You know, it's not really 100% clear from the datasheet, but looking at Figure 2-2, it does appear an external clock, I.E. a TTL osc can be input to OSC1, and you can select HS, XT or LP modes, so it "appears" you can use a TTL clock in on OSC1, and still make use of the HSPLL as long as you have HS set in config.

I'll have to experiment with that one.