Is there room for a couple of heucheras?
There's a small red leaved euphorbia, might be a variety of robbiae that doesn't run and is evergreen. It seeds around a bit but not so much you can't use the seedlings, and would make a nice contrast to hostas in the summer. Could probably find you a seedling if you PM me.

How about interplanting the hostas with grasses such as Pennisetums

There are some nominally-evergreen ferns that would go well here, they are wintergreen and you shear them back once new croziers start to form in spring, so if it were me I'd shear them just as the hostas poke their noses up!

I was also thinking maybe evergreen Ajuga, plenty are tough enough to cope underneath a hosta canopy - but then I thought of all the slugs that lurk under my own Ajugas...

How about the dwarf skimmia 'Magic Merlot'? It has evergreen, variegated leaves. You could also plant some cyclamen (both hederifolium and coum - that would give you two seasons of flowers, plus interesting foliage)

Hellebores maybe if conditions are ok

but if the hosta's over arch them then they wont thrive, I find the soil under the hosta in full summer is totally lacking any light. nothing not even the weeds get there root hold. I'd be tempted to go for winter bedding. or cyclamen hederfoilia that will peep out from under the foliage just as it dies back.