I have just successfully printed an iphone case with FlexPLA. However, I do not use the stock Velleman hotend anymore, I switched to an all metal E3D hotend instead, but I dont think there will be problems with the stock hotend either. If anything I would expect the PTFE heatbreak of the stock hotend to work better for this, as its more slippery than metal. Should you have jams or problems, many people “lubricate” the flexpla filament with a dab of vegetable oil, but wasnt necessary for me.
Anyway, I printed at 240C, despite the spool odly saying “<225C” on to a glass bed that was already coated with hairspray. It stuck MUCH too good to the glass, it took me 30 minutes to cut the object lose. Next time I will try some tape to print this on.
Also I printed at much slower than normal speed because I read that everywhere. I printed at 30mm/s and that worked just fine. I suspect I can print it a whole lot faster, and the reason you read to print this slow is bowden extruders where the rubber like filament can easily get stuck, but Ill have to try before stating this as fact.
Anyway, result is a nice, tough rubber like product.
As for your question, the default velleman nozzle is 0.5mm and the extruder hole ~3mm. I would most definitely not drill it wider, you would need to drill the PTFE too and that will ruin it for sure.
edit: oops, this is a month old, but better late then never ?