I don't believe a macerator necessarily means there was a holding tank - in "the bad old days" before we stopped tolerating water pollution, many boats' heads simply pumped out to sea. Could that have been the case with your Tanzer? It would explain the lack of a pumpout deck fitting.
You don't mention the size of your porta-potty or how often you have to dump it out ... but the biggest portable I've seen (mine) has a five-gallon holding tank. That's not a lot of used beer, but it's a lot of "stuff" to carry ... and to pour, oh-so-carefully, into the marina toilet. Bad enough after a long weekend, but every couple of days on a live-aboard...?
When I was shopping for my current boat, I saw one with a "super-potty" that might meet your needs. This was a one-piece unit, with a china bowl perched on top of a 9.5-gallon holding tank. (Dometic makes it, their 711-M28 marine sanitation system.) It would be almost as easy to install this as it would be to plumb a deck fill onto a porta-potty. (It does use fresh water to flush, but Dometic says it uses only about a pint per flush and that it has an integral vacuum breaker to protect your water supply.)
If you have a five-gallon porta-potty, this will hold about twice as much and need to be pumped out about half as often. It is, however, more than three times as expensive (West Marine 2009 wishbook: 5-gallon porta-potty = $160, Dometic 711-M28 = $569). And it's bigger - the tank is about 19"x19" square, and the bowl stands 20" high. (I saw one in a MacGregor 26X I was thinking of buying, and it didn't leave much leg or foot room in that boat's head compartment - but you may have more legroom in your Tanzer. I'd make a box and see.)
Maybe this will help?