So... back to system conversions.
The 'Daemonic Ammunition' entry in Creatures Anathema has an interesting mechanic for dissolving armour- might be worth adapting as an effect of the Alien's 'molecular acid' blood splash effects.
By the way (risking another derailment...), in the original post you were wondering why they used the term 'bug hunt' even though they didn't know the Aliens existed- what was the answer you found? I'm too lazy to Google it myself and wade through the results (-the real answer is that 'bug hunt' is a term used by Colonial Marines to refer to nuisance biomorphs, and not mega-threats like the Aliens; they were expecting a 'bug hunt', and instead found the Aliens- but Dark Horse Comics wrongly conflated 'bug' with Alien in their liscensed comics, and the rest is history; but I get the impression that you discovered something more interesting)...
Edited by Adeptus-B