Yeah... the android version is a very cut down version. So, unfortunately, there are a lot of really useful features missing.
I guess the dev's never envisaged someone trying to use the Android version as the "watch-only" wallet for an offline Desktop version

It doesn't strike me as an overly complicated thing to achieve (exporting transactions), so maybe raise a ticket on
their github and they might add it as an enhancement one day

HCP, you know, I think that is a very natural , self-evident, matter-of-course way if using the Android wallet version!
Look, on my online Desktop PC I want to send bitcoins to somewhere. They usually provide me with a QR Code which I would have just scanned with my Android watch-only wallet, export (save) the transaction file, and then signed it on my offline cold Storage notebook, then I'd again export it back to my Android (or PC's ) watch-only wallet for broadcasting.
That would be quick and fine!
I have never used the GitHub, yet maybe later I'll try to do waht you suggest