I thought I would come and report back on this. I attached foam cups to elastic and tacked them in and it was... fine.
But I have now perfected my knitted bralette and made one with a fold-over hem, encased elastic, and silk covered foam cups, made from the Jordy bralette pattern from Emerald Erin.
Planning to wear it under my sequined dungarees!
I am making one more, with matching shorts, this time with a picot hem and an opening at the centre back which will fasten with a standard bra clasp.