Hi John...
Your thinking is fine....
Have built up many circuits similar to what you need as well...
Steve on his Web Site has this circuit...
http://www.classeradio.com/vfo_2_band.pdfJust after the VFO is Q2 .... you could inject your DDS signal there and fiddle with the 50K pot....
The output doesn't have to be pretty but does need to be over 2 volts to change the state of the 74112...
On Jays Web Site W1VD he has a VFO there too using 7474 Flip Flops....
I use those ICs more often then the 74112....
You may have to add a little amplifer after the DDS just to bring the amplitude up a bit ....
Play around to get that part of the circuit reliable and the rest is quite easy...
Wayne
PS: as far as your 24 Fet TX goes I haven't built one using so many FETs so probably cannot help you there.
Steve would be the best bet and K1KBW has a WEB site that could be of some use as well....
Wayne