lol. That’s funny man.
Me and my mate used to do just this all the time when we were in last resort and could not, I repeat, could NOT get any bud for tokin.
So we did what your proposing, scaped up all the 'crud' as we called it from the inside of our bongs and down stems and dried it, mixed it with tobacco and smoked it. First of all it crackles like hash, but its just tar, and second, it tastes like absolute shit. not pleasant...
But it does give you some degree of a high, although wether its worth inhaling what you specifically removed via use of a pipe/bong in the first place is defiantly questionable. I would say it’s not worth it. Stick to fresh greens or some premium bubble hash or something and don’t go down that road of desperation that I, if not many, have done before.
Peace.