Looking at the 1.6 changelog got me interested in exploring PPSSPP's github. While nothing in 1.6 was fixed that would fix ridge racer networking, I did stumble upon this wonderful issue report on github from just two months ago:
https://github.com/hrydgard/ppsspp/issues/10698, which seem proposed a workaround to get RR2 adhoc working (?!). I have now tried this and this fix works for both Ridge Racer 2 & Outrun 2006 C2C! I have not tested others.
TL;DR: go to memstick/PSP/system/ppsspp.ini and set "WlanAdhocChannel = 1" (it is by default 0) on your multiple machines.
On machine 1, go to settings and ensure Ad-hoc PRO server is checked; put in the lan IP (127.0.0.1 might work just as well) for the PRO ad-hoc server address and hit "change MAC address."
On machine 2, go to settings and ensure Ad-hoc PRO server is /not/ checked; put in the lan IP (e.g. 192.168.1.2) of the machine running the server. hit "change MAC address" (just need to make sure it is different from machine #1).
Machine 1 must be used to host the wireless battle game (ridge racer 2) ---- or machine 1 must at least have network initialized (by going to the wireless play screen) & anyone can then host the lobby (outrun 2006).
Start the lobby before your slave machines search for the game.
RR2 supports 8 players, OR2006 supports 6.
I've tested both with up to 4 and they work fine.
RR2 is a bit annoying in that it is kind of finicky. Machine 1 must host, then machine 2 should search, try to join, and wait for acceptance. Then machine 3 should search, try to join, and wait for acceptance, etc. It's kind of a PITA, but it's all we've got for networked ridge racer and is far better than no networking. If you do anything more than "retry" you will need to repeat the process! OR2 is not like this and lets you change track, cars, etc. without having to re-create a lobby.
It should be possible to also race online instead of just LAN using coldbird.net, but I haven't tried this. It requires forwarding a bunch of ports -- more info is on ppsspp forums from what I've seen.
Enjoy.