It looks like the v5.5 PowerBook Edition may now be in stock,
https://store.inertialcomputing.com/product-p/scsi2sd-v5.5-2.5-inch.htm lists 22 in stock.
Inertial Computing may have something different to say, of course, but SCSI2SD is a relatively well-developed product, so there probably won't be a beta test of this new form factor. Although, the project does accept bug reports, so if you get one and find an error and can track down what's causing it or help troubleshoot, they'll incorporate fixes into new ROM versions.
The 2300c can be either IDE or SCSI from memory - so perhaps you were thinking of that?
I'd have to go look, but on the PowerBook 5x0 series, the way this played out is that all of the boards were SCSI and either a SCSI disk could be installed directly, or an IDE disk could be installed via an adapter/interposer board. This was, as my understanding goes, how the ~1GB disk configurations were set up.
It sounds like those adapters might not play well with IDE-to-something-else conversions, however, so your best bet on using one may be finding an IDE laptop disk from like 1994-1995.