I actually love the idea, just not sure as to how it will work out.
I see a couple complications in a few different areas.
Like someone had mention before, the person AFK or disconnected would have to be away for like 10min before a sub would be put in.
Problem with this is 10min is a long time for the apposing team to get ahead, but its better then nothing getting a sub at sometime.
Reducing that time to 5min I think would be better for one.
A large problem would be the sub. Getting masteries and a hero you might not be familiar with could be worse for the team then just playing 4v5.
I love the idea, but i can see some harsh consequences, but some great benefits if its implemented.
Maybe they could have some sort of voting system, might not be the greatest thing but still.
At the end of the match you care rate the sub from 1 to 10 on his performance as a sub.
When a sub is about to be summoned in, the team with the AFK can vote the sub in, or wait for the next one kinda thing.