Do you think this water saving mod worthwhile?

I don't doubt that could be done safely, but personally, I'm not going to do any modification that in any way connects waste water to the fresh water system. It's just too icky to consider.

Flushing doesn't use much as I'm sure you know. But I've not measured it. What I know is that I encourage using as much as reasonably possible when flushing. It makes for a well maintained black tank. I encourage DW to conserve in other places, not the toilet.