Well, holy crap. I think I just figured it out. Through trial and error I found out that not only is the subhood hidden, the lots are hidden too. EAxis really didn't want these to be found out. Anyway, here is what I did:
Open SimPE and go into your neighborhood folder of choice. I used Riverblossom Hills since that is my test neighborhood. In the neighborhood's folder, find the Hobbies subhood. Mine was numbered G001_suburb003. In my main neighborhood, the subhood ended in 003 too 004, but YMMV on the number. I don't know. Regardless, you can tell by looking at the Lot Descriptions once the file is loaded because the right hood will contain the names of the hobby lots.
To unhide the hood, I went to ID Number (IDNO) (under the Resource Tree) and changed the Neighborhood Type from Suburb to Downtown. Commit, Save.
Next I went to the Lot Description (LTXT), chose a lot (I used Sue's (Secret) Kitchen) and changed the U0 from 0x00000132 (edit: the number may be 0x00000133 for some lots) to 0x00000023. Commit, save. I can't claim credit for this part. I got the idea from Argon in this thread. I choose the 0x00000023 number because that is what the regular community lots seemed to use in the main hood.
I closed SimPE and fired up the game. Under my neighborhood chooser, Hobbies now shows up as a Downtown hood. The lot should be visible. I opened the lot, and used the cheat "changelotzoning community." Edited and saved. Then I entered the cheat "changelotzoning secrethobbylot."
I quit the game, reopened SimPE and reversed the changes I made to the IDNO and LTXT.
When I went back to test, the hobby lot was functioning correctly and my changes were still there. Stick it, EAxis. I'm not sure what would happen if you didn't rehide the lot and hood. I'll leave that for someone else to experiment with.
Addition 1: It appears that editing the hobby lot before it has been visited by a sim will cause the maven and visitors not to appear. To avoid this, make sure that any lot you edit has been visited previously.
Addition 2: Leaving Hobbies classified as a downtown subhood will cause downtownies to invite sims on outings there. Since the consequences of this are unknown, it may be best not to leave the Hobbies hood classified this way.
Please read through the rest of the discussion in this thread for more information about these issues.