Is there any scriptural basis for the proselytizing that has taken place in Buddhism?

Bhikkhu Bodhi has partially addressed this question - and coincidentally touched on socially engaged Buddhism - in an essay he wrote in 1993 and apparently revised in June 2005.

Tolerance & Diversity

He doesn't mention scriptural references at all. I don't know of any myself, but came across this quotation on a British website:

"Go Ye, O Bhikkhus, and wander forth for the gain of the many, for the welfare of the many, in compassion for the world, for the good, for the gain, for the welfare of gods and men. Proclaim, O Bhikkhus, the Doctrine glorious, preach ye a life of holiness, perfect and pure."

Buddhist mission?

Unfortunately there was no citation so I don't know where it hails from. I found the same quotation on the following website, which claims it was part of the Buddha's first dhamma talk.

Dhamma Talk

And I've seen it elsewhere, in slightly different versions, also said to be part of the Buddha's first public address at Sarnath. It seems to be the only missionary-like statement the Buddha ever made, or is recorded to have made.