This is simple to do and I've done it before with a few friends of mine on a Hamachi network of ours. Your friends won't be able to connect to you unless they are connected onto Hamachi, you yourself are connected to Hamachi, are a part of the same network you are in or are in a network you created, and you allocate your game to use your Hamachi address.
First, download ForceBindIP
here. Once you double click the executable, it will automatically install into the system32 directory. Next, right click on srb2.win and create a shortcut. Now, you'll need to change a few things with the shortcut. Right click on the shortcut and click properties. Type in the following in the target box: (C:\WINDOWS\system32\ForceBindIP.exe 0.0.0.0 "C:\Program Files\Sonic Robo Blast 2\SRB2win.exe") Copy everything in the parenthesis, including the quotations. If SRB2 isn't installed in a directory similar to mine, change it to accomodate your SRB2 folder location. You don't need to tamper with the directory for ForceBindIP as well, unless for some reason your operating system is installed on another drive letter aside from "C:" Also, replace the "0.0.0.0" IP with your Hamachi address.
Once that is done, load the game from the shortcut. If you want to use the SRB2 net launcher with it, set the default .exe to your newly created shortcut. And that's it! Your friends won't have to do anything other than be on Hamachi themselves. They will be able to join by IP simply by entering your Hamachi address in the "Join game <IP>" menu.
Edit: Also, be sure that port 5029 (TCP/UDP) for SRB2) and ports 12975 (TCP/UDP), 32976 (TCP/UDP), 17771 (UDP) for Hamachi are forwarded. This method worked for me like a charm. One of my friends, however, tried to host and do ForceBindIP himself but the process simply wouldn't work. Yet, he could still join my games. He has Windows 7 so that might be an issue or not unless he got the wrong version.
You can host with your public IP as well just by forwarding ports 5029. Go to whatsmyipaddress.com to get your IP and give it out to whomever you plan to play with.