I have no idea if this has been done before, but i've found a way to play LSPDFR, co op with friends (or publicly) with little drama.
Currently, i can host a my SP, LSPDFR world and my friends join me via hamachi (port forwarding is possible too) and the mods work fine (except for OpenIV installed mods, although im working on a workaround).
Currently, me and my friends can join and do our thing, however LSPDFR doesn't like it when one person pulls someone over and another tries the arrest. This is understandable, LSPDFR isn't designed for MP. However, if I follow my mate around like his code 2 backup (we have a rule where whoever initiates the pull over has to be the one to finish it), we have no issues. There were latency issues at first, but this was because the server was syncing our client side traffics simultaneously, causing long catapillar effects of cars. Ive sroted this temporarily by only allowing my own traffic/peds to sync with the server. my mates have to tell their clients via GUI to not sync peds/traffic. Ultimately, this means we have to stick close. However, if we don't want to work close, we can both enable our traffic/peds and then we just avoid each other!
I'm currently testing server side traffic generation, this SHOULD alleviate the traffic bugs.
Btw I've not done much work here, I'm using readily available mods which we can all just download, and only a small amount of network magic/trikcery.
IF I find a stable way to enjoy LSPDFR with a group of friends and its as bug free as possible, I will share what I've done to a T, contact the authors of the respective mod(s) and see what I can arrange to give more people access to LSPDFR co-op.
In summary, I know LSPDFR is not designed for co-op or MP, and probably won't be any time soon, if at all. Seeing as the whole MP talk died down about 2 years ago, i thought I'd try again with the more updated mods and new releases. Again, I don't want to fully disclose what I've done (although i'm sure some of you probably have guessed), but in short, I've used modded Minecraft "Open to LAN" logic and trial and error XD
Will keep you posted on my progress 🙂