I picked one of these things up because, well, for <$3 who doesn't need USB to TTL serial? 
I was reading over the Duo System dev notes, and I couldn't really find information on where the data pump is mapped into the address space. It looks like disassembly of the datapump driver might be the most expedient way of getting the address mapping...
JFYI, for disassembly, I've been using the fdisasm tool from minivmac. It allows distribution of annotations separate from the ROM to allow collaboration without distributing the copyrighted material, which I kind of like. It took a little bit to setup the minivmac instance and learn the meanings of the various files it uses and their syntax, but once I got the hang of it, things have been going well. If you're interested, I can probably setup the annotations for the driver's entry points.
I've been using it for a different project on the IIsi ROM.
FWIW, you don't NEED declroms... If you're going to occupy a nubus slot or pseudo-slot address space, you probably should. Outside of that, your declrom probably won't be probed anyway. If you're fine with hardware just sitting there not doing anything unless you have the proper software installed, you just need a non-conflicting address space to occupy AFAIK. You could probably even use the RAM expansion port and have a device occupy the top end of the RAM expansion address space (leaving a dead zone between the top of whatever RAM you do have and your device to help foil detection).
I was reading over the Duo System dev notes, and I couldn't really find information on where the data pump is mapped into the address space. It looks like disassembly of the datapump driver might be the most expedient way of getting the address mapping...
JFYI, for disassembly, I've been using the fdisasm tool from minivmac. It allows distribution of annotations separate from the ROM to allow collaboration without distributing the copyrighted material, which I kind of like. It took a little bit to setup the minivmac instance and learn the meanings of the various files it uses and their syntax, but once I got the hang of it, things have been going well. If you're interested, I can probably setup the annotations for the driver's entry points.
I've been using it for a different project on the IIsi ROM.
FWIW, you don't NEED declroms... If you're going to occupy a nubus slot or pseudo-slot address space, you probably should. Outside of that, your declrom probably won't be probed anyway. If you're fine with hardware just sitting there not doing anything unless you have the proper software installed, you just need a non-conflicting address space to occupy AFAIK. You could probably even use the RAM expansion port and have a device occupy the top end of the RAM expansion address space (leaving a dead zone between the top of whatever RAM you do have and your device to help foil detection).