I was messing around with my Flash0 after installing 3.72 HX firmware on one of my PSP. Since I read it's basically the same as 3.71 M33 I was very trusted. I ended making the 3.72 HX firmware to run 1.50 kernel'ed applications. So here's a tutorial how to make it.
Warning - You need to manage the Flash0 manually, be extremely careful -Warning
You need files from the Flash0 of 3.71 M33 firmware. Make a back-up before installing the 3.72 HX
Creating Flash0's back-up:
1. Go to the Recovery Menu
2. Select Advance
3. Connect your PSP to your PC using a USB Cable
4. Toggle Flash 0
5. Copy the folders to your PC
Install 3.72 HX-1 Firmware
Flashing your folders on 3.72 HX:
1. Go to Recovery Menu
2. Select Advance
3. Toggle Flash 0
4. Copy the folder /km to the root of the Flash0
5. Copy the file reboot150.prx to /kd/