(This is already possible with existing mods, see bottom)

Basemental's mod Universal Venue List allows players to select custom lot types (venues) from multiple mods.

The issue is that the choices a player is able to select is moddable, but not injectable (due to SimData), meaning that players will not be able to select venues not in the list. So, one big mod has been invented which puts the custom venues in all lists.

The venue lists talked about on Basemental's Universal Venue List page are not actually the VenueList tuning snippet, but rather the compatible_venues tag in a Region tuning. This means different regions can have different restrictions on what venues can be placed there. However, the game already has some "incompatible" venues. Sixam from Get To Work (region_Career_AlienWorld I think) is supposed to use the venue_HiddenAlienWorld venue for the lot, but the compatible venues list does not include that.

After scouring the decompiled Python code in venue_service.py:

This seems to change the venue type of the lot given by zone_id to the venue type given by active_venue_type. It appears source_venue_type and zone are unnecessary parameters; the code figures them out from other information.

Instead of a Universal Venue List mod that has to be manually added to by the creator, we might be able to construct a mod where a simple cheat could
change the current lot type. This would skip the Build/Buy UI, allowing any venue type to be selected, including normal, hidden, and custom.


Method through TwistedMexi's AllCheats Mod

Using TwistedMexi's AllCheats mod, you can use the cheat "venues.set_venue [venue tuning name]". This doesn't update immediately, but if you save and reload your game, it will take effect. You can now edit the lot as the custom venue, and it will save your changes. The build mode UI behaves well in this scenario. The lot type dropdown is grayed out with the correct venue type name, and hovering over it gives a notice that it is disabled for special venues. To be able to change it, just use a normal venue id in the cheat like "venues.set_venue venue_bar" and then you will be able to change it again.