Worst case scenario is the weld breaks out, washes down the the return port to the bottom of the oil pan and gets stuck in the pickup screen or makes its way threw the pump and gets stuck in the filter.

I don't think my weld will fall out, I've been doing stuff like this for awhile. I can't vouch for others aluminum welds though. :P

The easiest without a welder would be the right size ball bearing and press it in or push it in and punch the rim to keep it in. Other option would be to drill the hole out a bit bigger and all the way threw then tap it and plug it.