Success! (I think!)
I tried again with the transfer, this time using a glossy page cut out of a magazine, instead of the heavy photo paper. I also took more care to clean the board, and was more patient with the transfer process. Here's how the traces came out before etching:
There was 100% toner transfer -- the magazine page had white spots (bare paper) where the toner transferred off. Perfect! However, I did notice a few spots where the toner "smeared" a bit (likely due to some overzealous pressing with the iron). I touched up the pads and traces as best I could with an etch resist pen, then off to the etching. 30 minutes later, here's how it looks:Just from the looks of it, it looks like it came out great. But close inspection of some of the 10mil traces suggests that the etchant may have come under in a few spots, creating gaps in the traces. Hence the asterisk -- I still need to go through and do a continuity test on all the pads. Hopefully the traces that are broken will be repairable by simply tinning lines with solder.