Just out of curiosity, did you try that "maybe it *is* TTL?" 9 pin to Multisync (via resistors) idea I floated over on the VCF for working with the add-on video card?
As for driving one of those scaler boards from a Mac motherboard, well, two issues.:
Per this page about driving a Mac monitor from a VGA card, one of the lines that comes out of a Mac motherboard is a "Horizontal Drive", not a standard "Horizontal Sync" signal. Roughly speaking, the issue is that the CRT inside a Mac isn't a "complete" monitor and the motherboard compensates for that, and as a result it doesn't output a standard Hsync signal. Someone better versed in television technology would have to come up with the solution for this; my guess is that a fairly trivial amount of circuitry would be required to reshape the pulse appropriately but you'll need to do that in order to have a different monitor work properly with it.
(If you have access to one of those "Build your own Mac and save a bundle!" books describing how to build your own Hackintosh it's possible the requisite circuitry might be in there?... *googlegoogle*... No, annoyingly not. The book covers a number of different solutions, the one that involves directly connecting your Mac motherboard's video output to an external monitor is a "Power R" video adapter which has the necessary electronics on it; the book doesn't include the schematic. Hacking instructions for DIY'ing apparently originally appeared in a 1989 series of"Hackintosh" articles in Computer Shopper magazine. And finding the full text of those articles seems to be roughly as easy as finding the Holy Grail.)
The other thing I wonder about, reading the PDF instructions for the particular scaler you have, is if it actually supports "EGA", as in, "The frequency range that's roughly in the ballpark of a compact Mac's monitor.". The PDF says it supports signals in the range of "14.5-16.5K, 23.5-25.5k, and 30.5-32.5k", and none of those ranges *quite* cover the scan range of the Compact Mac's monitor. (Nor to they *quite* cover 350-line EGA.) If that's not a typo and it really doesn't support 21-2k-ish frequencies than it's not going to work with a compact Mac motherboard's output. (Is "23.5-25.5k" an Arcade board standard, maybe?)
As for driving one of those scaler boards from a Mac motherboard, well, two issues.:
Per this page about driving a Mac monitor from a VGA card, one of the lines that comes out of a Mac motherboard is a "Horizontal Drive", not a standard "Horizontal Sync" signal. Roughly speaking, the issue is that the CRT inside a Mac isn't a "complete" monitor and the motherboard compensates for that, and as a result it doesn't output a standard Hsync signal. Someone better versed in television technology would have to come up with the solution for this; my guess is that a fairly trivial amount of circuitry would be required to reshape the pulse appropriately but you'll need to do that in order to have a different monitor work properly with it.
(If you have access to one of those "Build your own Mac and save a bundle!" books describing how to build your own Hackintosh it's possible the requisite circuitry might be in there?... *googlegoogle*... No, annoyingly not. The book covers a number of different solutions, the one that involves directly connecting your Mac motherboard's video output to an external monitor is a "Power R" video adapter which has the necessary electronics on it; the book doesn't include the schematic. Hacking instructions for DIY'ing apparently originally appeared in a 1989 series of"Hackintosh" articles in Computer Shopper magazine. And finding the full text of those articles seems to be roughly as easy as finding the Holy Grail.)
The other thing I wonder about, reading the PDF instructions for the particular scaler you have, is if it actually supports "EGA", as in, "The frequency range that's roughly in the ballpark of a compact Mac's monitor.". The PDF says it supports signals in the range of "14.5-16.5K, 23.5-25.5k, and 30.5-32.5k", and none of those ranges *quite* cover the scan range of the Compact Mac's monitor. (Nor to they *quite* cover 350-line EGA.) If that's not a typo and it really doesn't support 21-2k-ish frequencies than it's not going to work with a compact Mac motherboard's output. (Is "23.5-25.5k" an Arcade board standard, maybe?)