This works! Rated 4 out of 5 stars
This add-on did what I wanted it to do, albeit only when I installed azurewelkin's fixed version for Thunderbird v13 (see the comment just before this one).
Some history: Somehow the domain-based automatic identity selection had stopped working at some point, perhaps on an automatic upgrade to Thunderbird v13. Previously I was able to reply-to a message, and my stock Thunderbird would properly select the identity based on the domain of the original message's recipient, not just the entire email address. This more "fuzzy" matching meant that if I received a message at "email@example.com", replying would automatically select my identity for "MarkerB@customdomain.com"
However, the add-on does need to be trained first, by editing the "detection" option for each identity I want to autodetect the domain. I simply select the identity, and add the corresponding domain as an alias. For my example above, I would select the identity for "MarkerB@customdomain.com" from the pull-down, and enter an alias of "customdomain.com"