I don't really care for the open stage phones - they are too "flat" for my and not comfortable to dial. I have a spacer block under the back of mine to give it a more comfortable angle. Do you think they can still use the optiPoint 500's on there? You can probably use an Optipoint 500 standard button table layout with the optiset Standard, Advance, and Advance plus because they are pretty similar 12-button layouts - the phones are also reasonably priced if you get refurbs. I still wouldn't think the expansion modules would work though, although you never know....
I know I'm in the process of putting optiPoint 500's on a Hicom 150, which is a step under the 3750 I think and they have you define them as optisets - for the 500 Advance plus you program 12 buttons on the phone template and the lower 7 on the first key module to get functions on all 19 buttons.