Just to get this out of the way now, I entered a team and am very happy to play with the current rules, but since you mentioned it was something you'd consider turning into a progressive I figured I'd put my two cents in for what would make it more viable long-term (I'd also be interested in something like that, I really like these sorts of themes).
I jrealized I'm actually 156 games into a league right now that is applicable to this. The league called for owners to pick one season and build their entire team using players from that season. Five teams chose teams in what I'd call the deadball era (1920 or before, understanding there's no perfect cutoff). Here are their current number of HRs allowed and position in their division (they are all 1-5, with 6th place having given up 117):
1908 (49, 2nd to 1920)
1909 (53, 1st)
1912 (40, tied for 1st)
1919 (19, 1st)
1920 (54, 1st)
1919 holds the best overall record in either league. 1920 has the second best.