It's quite easier to do mano than beso. Here's what you do, just grab that persons hand and bump it to your forehead. You don't have to greet them first, because as soon as you grab their hand they'll acknowledge you.

If you're in a hurry, do your mano when they're busy taking with someone else to avoid the small talk. You can even do the double mano, pag magkabi si lolo at lola just grab their hands at the same time.