Hmm I am not sure since I haven't had to deal with this problem yet, I just installed Muvluv though I want to finish the current game I am playing first.
Is the /HA4@100291B0 supposed to replace the /WXXXXXX?
I know something you can try, not sure it will help but it is worth a try.
First, create a shortcut to your agth.exe. Then you all the necessary stuff to it, so lets say that you are using rUGP Ver 5.70.18. Then it should look like this (except where your agth.exe is):
D:\AGTH\agth.exe /C /HA4@100291B0
Then you start Muvluv Alternative. Now press CTRL-Shift-Esc (to open Task manager). Then you go to Show-Colums and check the box for PID (Process Identification). Then you should see the PID-number (usually 4 digits, sometimes 3 or 2) after each process. Look after the Muvluv process, probably rugp.exe. Check the PID number for it (and do not close Muvluv!). Lets say the number was 3232. Then you add /P3232 to the shortcut you made, so it looks like this:
D:\AGTH\agth.exe /C /HA4@100291B0 /P3232
And when this is done and saved you execute the shortcut, while Muvluv is running. AGTH will then hook onto Muvluv with these settings. And remember to NOT close Muvluv after you checked the PID, cause the PID changes everytime so when you want to play Muvluv you must first launch the game and then check PID and edit the shortcut to match the PID and then execute the shortcut.
It may sound troublesome but actually it is very easy and it has solved many problems for me, just takes 10sec extra to launch the game but who cares about that as long as it works. ^^
Hope this works for you!
Ps. About the parts in Task Manager I am not sure what things are called for you, since I got a swedish XP. But it shouldn't be difficult to find.
Ps2. I assume you have the lastest AGTH version? Version 2007.3.2 that is.