Have started some servers on Open Arena using the classic RAIL24 config. I have managed to get the full InstaUnlagged mod to run on OA and it runs nice and smooth, pretty much feels exactly like Q3. I couldn't get InstaUnlagged v1.1 to run on Open Arena though so it's running IU1.2Beta3 which means you will need to download the mod from the server. Simple though...just type /cl_allowdownload 1 in to your console before you connect and it will download everything you need.
A note on maps, the FFA server is using the standard Quake 3 DM maps from the Dreamcast map pack and a couple others, CTF is using random maps that are good for Insta that I like (including Q3CTF1 hehe), 1V1 is set to use htourney1 but just callvote whatever you want. Everything can be downloaded straight from the server so no need to go searching for files. Just again make sure you turn on auto downloading in Open Arena (/cl_allowdownload 1).
Other recommended settings to type in your console are: /rate 25000, /snaps 60, cl_maxpackets 125, /com_maxfps 125