I'd say wait until it's completed, then you can show photos of before and after etc. The DVLA are used to build from scratch kits, rather than re-panel kits and their forms reflect this, so "if you haven't completed it how can you say what part you've used?" On mine, and others that I know about it's been done after an MOT - hence the MOT station have MOT'd it as the donor car, so maybe that reassures DVLA that all is well?
If it can be MOT'd when it's back from paint, without interior (i.e. depending on whether you've removed the dash, dials etc.), then why not get it MOT'd and send the forms off to DVLA for re-registration? Send me a PM at that point and I'll help with the forms etc.
- tenpin