Its dependant on if you are truly working to muscular failure..?
If you are all likely hood would say that you couldnt squat three times per week. Then again Smolov programs involve squating 3/4 times per week.. So it is achievable if adequate rest is supplied and sufficient nutrients are involved.
personally I would say doing the 20 rep squats more than once per week is a little to much.
Personal oppinion however.
On the question "Are hack squats a worthy substitute for back squats in a 20 rep program?"
I would say Hack squats are a great exercise but you should do your best to do back squats, front squats, leg presses and Sumo deadlifts (not all of them but you get my drift).
Do you train at home? If so my best piece of advice is to go to a gym. Not a "... Fitness" but a proper, Mouldy walles, dirty carpet & Smelly equipment type place where there are like minded people who will push you. Not to mention, places like the one ive described usually have the most effective equipment.
INCLUDING A SQUAT RACK AND/OR POWER RACK.
Fitness Instructor and Strength Coach
Pontypridd South Wales