The rules for Availability state specifically that the DM can adjust the availability of item depending on the setting (ex. no matter the population of the city on a feudal world, you just won't find a lasgun as there simply aren't any).
So, I would say start with the basic availability of Extremely Rare. While cruising around the Expanse, you can only acquire "Mechanicus only" items from a very specific sub-set of people. A)members of the Mechanicus who B) have the item you want and C) are willing to part with it. Since Tech Priests don't start a dime-a-dozen (compared to the total population) I would say that is a very small sub-set to be looking for. This availability is adjusted normally for population, as the larger the population the more tech priests there will be who can have the item and be willing to sell it (same idea as anything else).
However, a Forge World is a special case. Since it is actually owned by the Mechanicus, you would expect a whole lot of tech priests there. Many more chances to encounter someone with the item you desire, and more importantly, part with it. This deserves its own bonus, independant of population size, reflected in increasing the availability of the item. It would be further adjusted for population as normal (larger forges would be more likely to have the item than smaller forges).
The OP asked for an opinion, thats mine. And the rules seems to support it.