I needed to replace the print head in my Pro-10 a little over a year ago. After scouring the interweb, I discovered that Canon Canada sold brand new print heads. Not cheap, but less than a new printer. I believe the Pro-300 uses the same print head (with different inks). If that's the case replacement print heads will be available from Canon for a while.
As far as refilling. YES! It is as simple as applying ink to the large inkpad opening. No drilling or plugs required. You will need to pick up a chip resetter; available from Precision Colors (USA), Inkpot (Canada) or Octoink (UK). The latter of which also sells empty Canon OEM cartridges that you can refill. I have two sets of PGI-72 cartridges with PC ink. As soon as any one cartridge gets low, I replace the entire set of 10. That way, the maintenance cycle does not cause another colour to drop below the replace level. Day to day printing is with PC inks. For my gallery work I have a set of Canon OEM inks that I swap whenever needed. As the ink is directly above the print head, there is no need to flush between ink cartridge changes, as the maintenance cycle after changing inks flushes any of the previous ink through.
I use my ColorMunki Photo (older version of your i1Studio) to profile my regularly used papers. No need to profile when I am using the Canon OEM inks as I use the paper supplied profiles.
Precision Colors has just been sold and are moving their operations to the USA, so that will make it easier and less expensive for you if you are located there.
Enjoy your Pro-10.