After finally managing to insert a page resource (the one the 2.50 RCO lacked, causing crashes) I finally managed to make a customizable topmenu_plugin.rco file which worked on 3.10
To use this, please use my RCO Editor. If you have a customized topmenu for 3.0x, you can open it and select Extract All, open this 3.10 topmenu and select Replace Multiple. Please note that you should have RCO Editor v1.13b - earlier versions had a bug with replace multiple function.
Also, I found another issue with the RCO Editor. If you're getting "Subscript out of Range" errors when using the Replace Multiple function - please delete the tex_transparent.* files.
This RCO is based off my "clean" 3.0x topmenu. Note that if you wish to remove some shadow/glow images, you may replace them with the attached "blank.dat" file.
It's now publically released. Feel free to post this file on other sites - enjoy!