I've got a Manitou Swinger shock I want to fit to my Trek Fuel and the only way I can get access to the SPV valve when the shock is fitted is to fit it upside down.

Aside from there being more chance of crud collecting round the seal, are there any issues with the shock being in the 'wrong' orientation?

It'll be fine.

as long as it doesn't foul the frame during swingarm travel I'd have thought you'd be fine. They're sealed systems anyway; unless it has a gravity dependent mechanism in it, orientation won't matter at all in the real world (crud collection aside, maybe)

Think about how your forks work and its pretty much the same.
As above, if its not fouling any of the frame then you should have no problems

I've got one of theese mounted the wrong way up because of space and access and it's fine happily it comes with the decals the right way up for how it's mounted.

As the lube gets airbourne inside the can I don't think it matters at all but keep the sanction clean which is a good idea anyway.

I thought it would be ok but thought I'd check. Will check the clearance issue but will go with it that aside. Thanks.