It would be a very nice feature, and I think many people would use it.
For instance: I find out I'm going to be stuck late at work at the last
minute, so I just go online and tell my dvr to record whatever show
I thought I'd be home for.
However...when I worked as a Customer Service Rep, I was told that
every time we sent a 'reset' hit to a customer, it was very expensive,
and to avoid issuing them if possible. It was expensive in terms of
the transmission of the signal up, expensive in terms of use of satellite
time, expensive in terms of backing things up, so that new activations
take longer, leaving installers waiting when they could be moving on to
the next job...
I don't know if they actually have the band width to be sending out
individual instructions to individual dvrs. And yes, I know they snuck
that commercial onto all the dvr's--but that was an "all dvrs" done
in the middle of the night (low band width usage time.)