Doc...I don't think it is a good idea. When your impeller goes it can kick the fins into the line...if you add a second pump driven input you'll just drive them into the exchanger and then have problems digging them out. Get a big perko filter and a straight line large diameter water intake line to it and a quick release impeller cover and you can minimize delays due to clog and impeller failures.
Your fear seems to be "what if there is a failure when I NEED the engine" and I would say that :
1. Engines ALWAYS fail when you need them or they wouldn't be on.
2. Get a big honking anchor and rode that is easy to deploy quickly for such situations.
3. Make the chances for "typical failures" minimal through good design & timely maintenance (i.e dual racors, clean tanks, no bends in supply lines etc.)
4. Have the right tools & carry plenty of spare filters,belts and impellers and know how to install them.