"Can I use the bathroom?"
I've always heard お手洗に行ってもいいですか but I suppose that was in the context of school, where you'd be asking permission to leave more than permission to use the bathroom itself.
I disagree on that. トイレをかしてください sounds rather rude, and I have yet to hear any native speaker ask like that. ください is used to form orders not request. Yes in a polite way, but using it implies you are not expecting a negative answer. The form in ていいですか seems way more suited to me, as we're asking for a request.
I was going to ask this as well. When Looked in my dictionary, I found what look like related entries:
貸す, 藉す [かす]: (P, v5s, vt) to lend, to loan, to rent out, to hire out
仮す [かす]: (v5s, vt) to grant, to allow, to forgive
Does anyone know if 仮す could also be used?
Is this just the way it's said in Japanese? Like one of those situations where it's not a direct translation?
Yes. There are other ways to say it, as well, but this is a common way to ask.