Would a concealable holster's bonus stack with that of a lined coat?

If so, does the lined coat's +50% concealability apply before or after the holster's bonus?

Yes. The +50% is to the final Concealability of the item.

I find it's easiest to add the +2 for a concealable holster (or, for that matter, any penalties for a silencer, larger clip, or any other external extras) before applying the +50% for a long coat. I'm not sure if there's a canon answer as to when it's supposed to happen.

Probably before, but as long as the GM picks one and is consistant, it'll be fine.

It inadvertantly answers it...kind of.
(Rhetorical: Man they missed some things in this book didn't they?)

SR3, p237, Manual Detection:
"In all cases, make a standard Perception Test against the Concealability of the weapon in question. Apply the modifiers in the Physical Search Modifier Table, p. 236."

Since they list the conceal rate of the weapon first, and then the modifers second.
I'm guessing the angel you look at it from.

It's a reach, but they didn't say much else that I can find.

But I'm also not sure if it matters what the order is off-hand.

Here's the logic. It's really simple.

You place the gun in the holster (Conceal +2). The holster and gun is now hidden beneath your lined coat (Conceal +50%).

If in some strange world you were wrapping your gun in your lined coat (+50% Conceal) and then somehow shoving it into your holster (Conceal +2), then this newfound ridiculous situation would be reversed.

Why am I envisioning a tiny lined coat that a guy has hanging from a strap on his chest? And why do I find it endlessly hilarious?

oh my dear living hell...that was just too fricken hilarious