Wel, it might be possible to create a PCB which acts like a replacement to the normal Sony-NWA (like those Fake-NWAs with S-ATA do), but those still use some kind of controller-chips, so...
I don't think it is possible without an extra controller-chip which 'answers' to the PS2: 'Hey, I'm present/connected!'.
PS2 V7/DMS3 V2 (FW:2.4b7); Seagate Baracuda 200GB
PS2 V7/CC1.0 (FW:34 hacked v2 BM:2.1.6); Maxtor DiamondMAX9 PLUS 160GB
PS2 SCPH-30004R; NoMod+NoLaser
3xSony MC 8MB
MAX/Datel 16MB with Boot-CD
Custom FMCB 1.8b+ Beta-Build, my AIO 0.5, Sony&xRhino-Linux