Sandy, they're handled the same way in OZ and EU as they are here
Except that holding tanks aren't as universally required in either place as they are here. And the laws vary a bit from ours. For instance, in OZ you can dump a tank as close as 1 mile offshore if it's macerated. All of Turkey's coastal waters are "no discharge" but because there are -0- pumpouts in Turkey you have to go out into the Med far enough to be outside Turkish waters and dump your tank when it gets full. The Turks believe in "sharing the wealth" with all their neighbors.
What I think you're overlooking is the difference in the number of flushes from various toilets. Portapotties use so little flush water that 5-6 gallon portapotty--including those that can be permanently installed and fitted for pump out--can hold 50-60 flushes, which is enough to last a two people continuously aboard at least 5 days. Otoh, the average flush volume from the typical manual or electric marine toilet is about 1/2" gallon...which means at least a 25 gallon tank is needed to hold the same number of flushes.
As for the cost..2-3 gal portable portapotties are $100 or less...an extra tank prob'ly around $50. 5-6 gal. "MSD" models are <$150...add a few feet of hose and fittings for a total of +/- $200 if you do the work yourself. A typical holding tank installation would run <$500 for the tank, hose and fittings if you do the work yourself...whch is actually not that challenging because holding tanks (or treatment devices) have been required since 1980, so you'd most likely be replacing instead of starting from scratch.
So what it comes down to is, there's really very little need for a "swap out" service...'cuz a 5 gal tank connected to the typical marine toilet would barely last two people a day--not even that long if they have even one more person aboard...so nobody would be willing to put up with the hassle of swapping out tanks that often even if they had several on hand. And a 5 gallon portapotty holds enough flushes to last till the boat has to come in for ice or fuel (beer?) anyway...and can be pumped out then. If you have only a 2-3 gal. portapotty, a spare tank for it is relatively inexpensive and easy to swap out if you want to stay out long enough to need it.