I glued the label on tonight. The process went well.
I changed my mind in the end and decided to use contact adhesive for a cleaner job.
I pondered the method for a while and came up with the following:
First, I made a mask the size of the soundboard, with an oval cut out of the centre for accurate positioning. I made the oval cutout about 1mm larger all around than the label:
Attachment 267092
Next, I made a second glue mask, that I aligned and taped over the top of the first:
Attachment 267093
This one was slightly smaller than the label, so I wouldn't get glue outside the label borders. I spread glue through it, then peeled it off.
Now I spread glue over the back of the label, then positioned it inside the position mask and smoothed it down:
Attachment 267094
Next I peeled the mask off and voila:
Attachment 267095
The inner label was trickier, in a way. No mask, and only one shot to get it right.
I coated the back with glue, then, while the glue was still wet, quickly positioned it and smoothed it down. Lucky for me, it went pretty well. Not perfectly centred, but plenty good enough.
I had slight squeeze-out in two spots, so carefully removed it and again, voila:
Attachment 267091
(I even got it right way up. :D )
I'll let the glue harden for a few days now before doing anything else. I'll let it sit overnight, then use a 20W CFL lamp to warm the area for the rest of the curing time.
On Monday, I'll apply a coat of gloss Estapol over the top, then maybe a second a couple of days later. (I remembered to scuff the surface tonight before gluing, so it's good to go for the Estapol coats when the glue's dried properly.)
Edit: Another tip that I forgot to mention - to hold each label piece still while spreading the glue on the back, I used a pair of dividers. If the labels moved around, I might have got glue smeared on the front.
Also, before gluing, (a few days ago), I coated both sides of both label halves with spray Estapol, so the glue couldn't soak through and mar the front.