Both are tricky electrification jobs, particularly the latter. But 99s will be perfect for them, given their distance and speed.
I was thinking along the same lines about how the 99s might be perfect for aggregates traffic, and you've got there first.
What I was thinking (and may help people less familiar with the MML, or spur further debate):
For example, Bardon - MML is downhill and basically walking pace, so it can't need much power.
Then onto electric where you'd hope to see a significant performance benefit on the climbs out of the Soar/Welland/Nene/Ouse valleys
Back onto diesel for the Dudding Hill line (slow), and a crawl through South/West London behind stopping services to a final destination like Angerstein Wharf.
There's basically no heavy freight south from Leicester in the daytime, and I am guessing this is because they cannot get to Kettering without holding up an IC. So they run overnight, or go via Melton/Corby (if originating at Mountsorrell or further North).
If a class 99 has enough oomph to go via Harborough without delaying EMR, then doing so saves mileage, time and releases a bit of capacity between Syston and Manton Junction
Coming back the other way, it's all empties so clearly power is less of an issue (there's Northbound freight trains even during the peak periods).