Not a big problem. Do what you can. Reverse grip preachers would be another option. I'd also consider changing the rep range. Things like hammer curls or pinwheel curls put you in a good leverage situation to work heavier, something that's tough in the 6-8 rep range. Keep an eye on not using too much body English, but do try to move some weight.
If you happen to have Fatgripz or something to make the bar thicker (I'll sometimes put two bar pads next to each other on a barbell to fatten it up), that could also work. Using a thick bar for almost any exercise will increase the forearm/gripping work.
Ah, tricky situation where I probably should've clarified. Reverse-grip curls are done palms down, pronated. This increases the forearm contribution to the movement. Reverse-grip rows are done palm up, supinated, which increases the biceps involvement.
I meant to suggest finishing back day with some reverse-grip or neutral-grip curls, just a few sets after the back work for the day is done. A reverse-grip row could be good for more general arm involvement, but wouldn't necessarily hit the forearms a ton better. Though neutral-grip rowing/pulling does seem to bring more forearms into the action.