Some more interesting info .
The firmware for the Crestron HD-SCALER-VGA-E contains a reference to Cypress (CYP) and to the CP-293N . That would help explain why the HD-SCALER-VGA-E accepts 1024x768@70Hz (just tested), even though it is not documented in its manual . This resolution happens to be the only 70Hz one supported by the CYP CP-293N, according to its manual . As mentioned before in this thread, I also suspect 1024x768@70Hz input will work on the Startech VGA2HDPRO2, even if it is not documented as I believe it is probably similar to the CP-293N .
However, as none of these scalers support 640x400@70Hz or 720x400@70Hz as input, so they are not really useful to us .
I disassembled my defunct Gefen VGA To DVI Scaler Plus (EXT-VGA-2-DVISP) and I found a sticker mentioning CP-252GM v4.0 on a socketed W79E632A40PL micro-controller with flash that I was, unfortunately, unable to dump (TL866II Plus does not support it and it is probably read protected anyway) . Now this may have absolutely nothing to do with CYP and just be a huge coincidence, but considering that the CYP CP-252 just happens to be a VGA to DVI converter that supports 720x400@70Hz as input, just like Gefen VGA To DVI Scaler Plus, I have my doubts . To be fair, the Gefen unit supports only 60Hz output whereas the CYP does 70Hz at 1024x768 and 1152x864 .
The more I look, the more I see roads that seem to lead to CYP in the VGA to DVI/HDMI scaler market . Not that this is a bad thing, as CYP's stuff seems well designed, built and functional (and they have a very responsive sales department) but it does imply that there is probably much less of a variety of products on the market than one would originally have assumed . Also, if CYP does not happen to currently make a product with a feature you want (e.g. 640x400@70Hz scaling to 1600x1200@70Hz), chances are nobody else does (not taking into account OSSC).
EDIT: Corrected typos