Following on from Fae and myself meeting Robin Urquart of the National
Archives of Scotland, I'm looking for people who may be interested in
working on a WW-I related GLAM project.

This is very interesting stuff - thank you for taking it on. :-)

Here are a few thoughts...

I imagine both Commons and Wikisource would be keen to have the material available. However, Wikisource in particular doesn't have a community that will just get on with transcribing letters. So you'd need to be careful that NAS don't expect that they will get a whole load of transcribed letters for free. 

I'm not quite sure what Wikibooks would think about this kind of thing - I think it's more aimed at textbooks than source material - but you might as well ask and see what you get.

Since Wikipedia is by far the biggest and best-used project, finding a Wikipedia link is probably quite valuable in terms of impact. Since 99.99% of the individual soldiers are non-notable, you could approach this by using materials for the war memorials as Andy suggests, or with reference to the individual battalions (we actually have poor coverage on Wikipedia of things like war-raise battalions), or perhaps the towns. 

Due to
the accessibility requirements imposed on any body like the archives,
there is a need to transcribe such documents before they can make them
widely available.

Obviously if they use Wikisource as a transcription tool, then they will need to make the documents widely available, and then transcribe them. :-)

It would be really interesting to see a GLAM initiative which used Wikisource as a transcription interface. There are plenty of non-Wiki GLAM initiatives which have custom-built transcription methods (e.g. Old Weather) and Wikisource has strengths but some disadvantages as well. 

But, I've a hunch this is
something that could be excellent for waking the wider public up to
projects other than Wikipedia, recruiting local history buffs as new
content contributors, and getting cultural institutions to 'think
outside the box' around working with us.

Yes, this is why WWI is a big opportunity. :-D