ok i got it wrong...
still, i think i would have a look at what inkscape can do and would focus on how letters can shrink, bend, and so on... without aiming at automatic interactions between the items.
processing (or shoebot) might also worth to be considered.
finally, i made a short test and, indeed, text on path does not "flow" but text on path already has so many parameters to take into consideration, that i'm not sure how good are the chances to get a nice result, when letters on a path move around because of of "conflicting shapes...
i'm not sure it's worth a feature request...