The Alpha Legion do, in fact, do that. Or, at any rate, they did something like that at least once. And there's always a chance the Blood Ravens have been doing just that.

 

It'd be tough for any length of time. They still have to send their tithes of prognoid glands to Terra, though I guess if they slaughtered all the Loyalist marines in a chapter they could keep that up for a while. And place any unneeded ones in stasis to keep'em fresh for the next round of tithes.

 

The Black Dragons seem to have come up with a way of cheating the geneseed tithe tests so I don't think it's such a hard thing to do.

Plus seeing as how many many anti-Imperium alien races exist in the WH40k universe with tech that could easily fool the Mechanicum, I don't think finding one willing to pass on some of their advanced tech in exchange for something in return is an impossibility either.

 

It has to be hard enough since a lot of current Chaos or renegade chapters usually stop turning in their tithes before tipping their hands.

 

For a short time? Yes. Depending on where they are, maybe even a not-so-short time.

Many PDFs and planetary governments won't probe too deep into Astartes issues. there's no polite way to approach a hulking born-murderer and go "So, you mind proving to me that you're actually a servant of the Emperor and not a heretic?" without taking bolt rounds to the face. Also, remember that very few people actually know about the Traitor Legions with any degree of detail: ignorance of evil is very much the norm in the Imperium except for Inquisitors, IG generals and a few others.

Add the slow communications and response time of the bureaucracy, and a warband of the Alpha Legion or Night Lords could well stay undercover for decades or centuries in some sectors.

 

You have to remember this fact: Whilst we see Astartes on the tabletop and in the fluff all the time, the majority of the populations in 40k fiction will never even get a glimpse of one in their lifetime.

This is why you often come across situations whereby the planetary defence of a world will hesitate when a Chaos Astartes vessal appears, because they are so rare that no-one really believes its there, and just assumes it must be a loyalist untill they scour their vessal records and see the terrible words 'traitorous excommunicatus' and proceed to sh*t themselves.

But, back on topic, in the novel Lord of the Night, the Night Lord protagonist, despite his knack for disemboweling and crucifying people with their eyes gouged out, was actively worshipped by a group of civilians who thought he was an 'angel sent by the emperor'.

But, thematically, it would be best to make the Astartes in question be from the Alpha Legion, as that is exactly the type of thing they get up to.

 

Though it might upset some...

Didnt they imply in a novel that the Dark Angel Chapter were not loyalist...

i believe it was desent of angels...but i could be wrong been awhile

 

The answer to your question has been answered in the short story Savage Weapon by Aaron Dembski Bowden.

 

It might be easier, if a step waaaaaay too far for those prospectively involved, for a Loyal Chapter to feign becoming traitor/renegade. It would need heavy Inquisitorial support, to ensure that the Chapter can be supported in secret, and would need a Chapter who is rather independently-minded.
I'm not saying it's likely, not by a long chalk; but it's something that might be easeir to do. Feigning loyalty would, as has been pointed out, mean having the web of association to other Imperial organisations. It would only need one middling Beaurocrat to become suspicous and make some basic checks (who collected their tithes? how long have they been based where they are? how often do they appear in Imperial records? etc), to bring it all crashing down on the impostors. Although they might have a lot of fun before they're discovered...

GFP

 

The Space Wolves have been masquerading as loyalists for ages, so it's very possible. Filthy heretics.

 

ye, aswell as the SW, the blood drinkers and the flesh tearers are blatantly under the sway of khonre, they just blame it on the geneseed

 

And Blood Ravens are a bunch of Tzeentch worshippers. And perhaps the Chaos God of Retarded Commanders, but I don't know his name. Could someone with the Liber Chaotica look that up? Much appreciated.

Midnight

 

I really dont think it would be as hard as you think to collect geneseed tithes given the prevalence of the Ultramarines geneseed, cap and harvest a few marines here and there in your traitor persona and bingo.

Plus if the administration dont know exactly all the names and disposition of chapters, then sneaking one in wouldnt be too hard, lost in the warp, crusade chapter just claimed a world, really your sure your records are correct? all viable answers to a suspicious adept.

 

Astral Claws ring any bells? If not I'm gonna pull a Lux and say the UltraSmurfs.