I picked up an HD500 a few weeks ago and so far I'm loving it - I've mostly got to grips with how to build tones with it. It can be quite intimidating at first, but the PC software is a dream to work with so that makes setting up your tones that much easier.

In answer to your question there are a number of approaches you can take. You can create a patch with an echo effect in the chain, on or off by default. You can then assign the echo to a footswitch and switch it on/off as if its a separate stompbox within the patch. You can do this with multiple effects.

As for parameters being altered on the fly there is always the expression pedal - you could assign one effect parameter to the expression pedal. A bit limited, but it would work. Either that or just save your different OD setups as different patches.

It's really not that hard to go in and tweak on the board itself once you get used to it.