This would also create massive amounts of spam with people asking to be tipped for their shitty posts and people begging others who have been generous in the past
I don't want to be off-topic or controversial, but IMHO allowing paid signature campaigns already generates much more spam and shitty posts than the tipping incentive would. As DaRude says, tipping is a nice feature and practical use of Bitcoin, and abuse should be dealt with the usual way by moderators.
Generates spam maybe, but not another way to scam and not bigger incentive for begging, even from newbies and lower ranks.