Looking at this spec list -

http://www.darksidedevelopments.co.uk/octavia-vrs-2-0-16v-tdi-bmn/

Does anyone have an idea as to what is filled, and what the benefit would be?

I've only ever seen the solid 42 Draft Designs Solid bushes, never seen Poly ones anywhere.

 

http://www.42draftdesigns.com/VW-Shifter-Bushing-Set--Mk4-56S-MY02-05_p_279.html

Yeah I've never even heard of it either. Not sure what bits you'd fill either looking at the design...

I'm not familiar with the bushes in those, but I have heard of and seen told trick of filling up voids in bushes to stiffen them up, you get some of the 2 part liquid urethane that you mix up yourself with the hardener, then you pour it into a voided bush and let it set off hard, quite a useful technique especially on old rare classics you cannot get bushes for anymore and you pour it in to fix a broken bush