I don't see how we can get anything like WiiKey Fusion.
It's not possible without adding new interfaces to the PS2. Especially since the only external interface that all Playstation 2 console models share has a bandwidth of 12Mbit/s (USB 1.1 lol...).
Besides, SATA to IDE adaptors exist, so you can use SATA disks with the network adaptor. Not to mention that there are compatible Network Adaptors with a SATA interface instead of the old IDE interface.
USBAdvance offers poor performance, but have you tried PS2ESDL yet? It does give better performance, although it will usually never exceed the performance of the PS2's CD/DVD drive.
It has i.Link port support too, but device compatibility is poor because I don't have access to the hardware that it is currently incompatible with. But it is another method of booting games (As long as the console has a i.Link port).
Booting games over the LAN is another option too.
Unmodified SCPH-77006 with SM 3.6
SCPH-39006 with M-chip modchip, SCPH-10281 NA and refurb Seagate 80GB HDD
SCPH-10000 v1.00 with SCPH-10190 PCMCIA NA and SCPH-20400 HDD unit