Hi.  I am considering producing videos for youtube, and it would really help if there were key commands to trigger EPR, LNAV, and VNAV. Are there commands for these functions? :-/

As far as I know. No. There are no key commands for these. Captain Sim also use their own coding for the LNAV and VNAV buttons, not sure, but probably the EPR button too.

If you have a registered version of FSUIPC, you can use it with LINDA to create key commands (I think) and joystick/controller commands. I don't know much about LINDA, but there is a dedicated forum for it HERE.

OK.  If I press Z, will the default autopilot follow the FMC?

I use 'Z' to switch the A/P OFF, so I guess it could switch it ON. ;)

Thanks!  One more question.  Is there a kind of auto-taxi feature?  If not, are there any systems that might be able to guide me to the runway?

There is no "auto-taxi" feature for any aircraft that I know of.

There a the DBS Airport GPS that can be used to guide you from any Gate/Parking Ramp to any Runway, or from any Runway to any Gate/Parking Ramp at all airports (as far as I know). The only thing wrong with it is that it will not necessarily guide you by the taxi route that the ATC has given you. But like a real world GPS, if you miss a taxi turn, it will rebuild the route from you current position. It will not move the aircraft along the route, you have to follow it yourself. I bought it a couple of years ago and it's pretty neat and I like it. They also have a Follow Me program, but I don't have that.

FS Dream Team have GSX, which provides a Follow Me car, but I rarely use it. I do use the "Just warp me  there" feature occasionally, when I'm being lazy, to get me to the Gate/Parking Ramp without having to taxi all the way.