A popular strategy in pictionary/charades is, once you have part of a word or an idea correct, you point to that person/draw that word to encourage people to work in that direction.

For example, I'm drawing an air mattress and somebody says mattress, I want to be able to highlight that this is correct.

The simple solution would be for players to adapt a convention for drawing arrows at the person who said the right thing. A more technically difficult solution would be to allow the drawer to click on guesses (in the guess control) and those would become highlighted, to encourage players to guess in that direction.

If this sounds a bit cheap, (and I hope it doesn't) please note that in pictionary, from what I have been told, it is legitimate to write a word down as long as somebody has said that word.