I've used the splashtop app.

BUT, i found it very fiiddly (and I'm a geek with a positive attitude to doing this) and inaccurate (due to finger being the equivalent size of using a rounders bat on a smartboard)

I'd get hold of some capacitive pens and see how they works out.

Years ago when remote slates came out (Activeslate etc) - some teachers could really really use them to good effect (They had pens) but others couldn't cope with them at all so we moved to just using Smartboards (which anyone can use) but I still have one teacher who wouldn't be without her slate.