It looks like the least expensive and most DIY method would be to use the bulk of the Gert VGA666 design with an XOR gate to merge the sync signals.
I wasn't sure that we could configure the timings correctly until I found the following discussions:
See the posting by PR77 » Wed Feb 18, 2015 9:27 pm
It's not exactly the same resolution, but it shows that it can be configured for lower resolutions and a much lower dot-clock. If it can do 4.8MHz, it can definitely do 6MHz.
Timings parameters defined by dom » Wed Dec 12, 2012 1:39 pm
If you wanted to buy (instead of etch our own) Gert VGA666 board, they're only £6.00 (but we'd have to wait for shipping from the UK). I'm not sure if they're available from a more local source.
The XOR gate for merging the sync signals is just a normal TTL chip... should be really cheap to get.
From what I've read in the discussions, using this method of configuring the display will have the display default to this mode/resolution, even the multicolor splash screen comes up using the defined resolution and timings. This would ensure the monitor doesn't get overdriven during bootup.
Either way should work safely. I've simply enjoyed learning about all of the options I never knew we had before (especially the way the video actually works over the GPIO... but that can be another discussion).