Hi @randy and @brianvotoole,
I also goofed around with it.
I independently came up with the same concept that you did @randy using ARRAYFORMULA(), an expanding number of IF() statements and a REGEXREPLACE() to get rid of the final comma.
My version is:
={"Tags";ARRAYFORMULA(REGEXREPLACE(IF(P2:P,$P$1&",","")&IF(Q2:Q,$Q$1&",","")&IF(R2:R,$R$1&",",""),",$",))}
If you need more columns with Tags options add more &IF(?2:?,$?$1&",","") blocks where the ? is replaced by the Column letter. Reduce those blocks if using less columns.
I always add the comma at the end of a tag segment and then just remove the last comma at the end.
Jon