I think it's marvelous idea, as is the work done by Excalibur.
I'll throw in some coal fired stuff, as I've been in the coal fired power industry since 1988...
The coke residue could theoretically be used, but we've burned petrocoke before as an admixture, burned fine, but we had to turn off the metal detectors in the coal stream...meant that tramp iron then got through and broke stuff...it worked, but had too many hairs on it.
Rubbish in the coal stream, as others have mentioned as a possibility isn't...the occasional coke bottle thrown on by a lazy miner gets caught in the pulverised fuel mill classifiers, and blocks them, requiring an operator to shut it down, get a permit and fish it out...partial load reduction.
If a coal power station co-fires waste, it must have licencing and pollution controls as if it was entirely a waste burning facility, and all of it's ash treated like waste ash (hazardous), which limits it's storage and ability to be sold into alternative markets.
Where the oil produced would be REALLY handy, is in ignition support.
* 250,000L to start a cold boiler
* 100,000L to start a warm/hot boiler
* 1,500 to 2,000 every time a PF mill is placed into service (5+ times per unit per day to follow load).