I have, on an old HP 5L. It involved making a hole with a special soldering iron tip, disposing the leftovers in a plastic bag and then refilling from the toner bottle. The hole was then covered with tape. It was easy and the quality was indistinguishable to OEM.
I've refilled my HP laser twice with no problems. The potential for making a mess with toner is an order of magnitude greater than when refilling with ink, so do it in the basement or garage until after you have done it the first time (it's easy to get some of the very fine black toner dust into the air). I bought a toolkit that had a special tip for a soldering iron (included) that melted a hole through the casing (to eliminate chips from drilling) and a rubber plug to plug the hole after it was refilled.
I have read that you should only count on 1 refill per cartridge on average, as the drum will eventually become contaminated. Sure enough, some time after my second refill dark areas started to appear on one edge of each sheet. When this cartridge runs out, it will be replaced with a remanufactured unit to get a clean drum.
5L-6L cartridges are easy to fill. You do not need to make any holes in the cartridge. Get your cartridge and turn it on it's end. You will see a silver metal pin. Get a small screwdriver and push the pin in. Turn the cartridge up the other end and go the same to that pin. The cartridge will now come apart in two half's. Get your screwdriver and undo the end cap on the Non gear end of the cartridge and remove the cap. You will now see the fill hole and plug. Remove the Plug and empty the old toner out and clean cartridge. Put new toner in (140Grams) and put cartridge back together.