We have not thought about a web version, this was more of a learning project for Python/PyQt since these are not tools we typically use otherwise, but would be useful to know. We are also not aware of any simple way to turn Python (worse yet PyQt) into anything web based. If such a tool exists that would be interesting, rewriting it all is not all that appealing, at least not at the moment. Although come to think of it, it would be much more portable and probably work just as well.
Hopefully it should behave exactly like GT6 reactors, it took some crawling through GT6 source, hopefully there are no mistakes (we even made sure to use divup, so it really should be exactly the same). A simple test case also produced the exact same steady state neutron count so it seems to be working correctly.
As for the objects, adding fuel rods or coolants should be trivial and requires only an icon/color for the GUI and the corresponding data for the fuel/coolant. Adding utility rods (reflector/moderator/absorber) would also work, but at the moment the GUI treats the default 3 differently, so it might look a bit odd. That said, adding new utility rods or similarly new effects to coolants would also require a rewrite of GT6 code, so presumably that won’t happen. 