OK, looks like the new BIOS for customSH2 is ready. I have updated the first post with a new link.
I don't have a real CustomSH2 cart with me, so I can't test it on the real thing, but I'll ask @Mitsurugi-w and @djsheep if they can give it a go.
What does this mean? It means that now you can use the UltraSimm with any cart. You just need to update 1 chip ( the flash BIOS 29F400) in the cart.
Quick correction. There are two versions of NoCD one for custom SH2 and another for standard SH2. However as the name says, none of them allow to change game using a CD. To change game using CD, you need to use the BIOS that says CD in the filename and is the right version for your SH2 processor. Either one will allow you to change game using CD but only if you use original CPS3 simms, not UltraSimms.
A CD version of the BIOS will always require a CD / SCSI2SD present as @Mitsurugi-w said.Just to clarify then, does a CD / SCSI2SD need to be present when using the CD versions of the BIOS with pre-flashed UltraSIMMs? Or does it function the same as the NoCD BIOS in that instance?