If you're certain this set wants 1.5Vp-p RGB delivered to the neckboard, I would expect something to show up at .7V, and your problem could well be with sync.
crt_emudriver 1.2b doesn't have the ability to enable c-sync in VMM, does it? Calamity recommends you use the updated 2.0 tools with the 1.2 driver, but I know there are limitations with that. I would guess it's not going to work. You might still be able to enable c-sync with an older version of Catalyst Control Center or another program, but there's no official compatibility with the driver. It's going to be trial and error. VGA delivers TTL sync with a 5V DC offset, so you're measuring that. If your multimeter has a frequency setting you can mb use it but it will probably only register the H-sync component at about 15kHz and ignore the V-sync bit.
You can try a 500R resistor followed by a 1K pot in the V-sync line, and the same in the H-sync line, then combine. Dial the pots around and it should work, assuming you're injecting sync to the right place. That being the composite video port, or equivalent pin on the jungle, if you're doing this in AV mode. If/once you get it working, you want to be using the highest value resistors you can get away with.
Be aware that injecting RGB to the neckboard is not the preferred method as it bypasses the regulation circuitry in the set. You normally need to amp the video levels as you describe, but if you use a pre-80's set, and/or if you make the picture brighter than it could go with the RF/composite inputs, you'll be getting an upped dose of X-rays. And you stand far closer to an arcade tube than when watching TV on the couch.
You might consider modding the set to inject RGB into the jungle, if it's possible. There's a thread about RGB modding in the Hardware section on the Shmups forum, if you need pointers.