I wrote an e-mail to Heiland with my questions and promply got an answer the next day (from Juergen Heiland himself). So, good service!
I'll add the info here for Photrio members searching on this subject.
1. The USB option (DE-OPT.03) for the Heiland Heiland TRD2 densitometer can be used independently from the DensePC software.
2. The USB output emulates a RS232 (COMx) connection and sends data out automatically after a stable measuring value has been detected.
3. The data format is described on page 15 of the user manual TRD2_Uman_V64.pdf which can be downloaded from the Heiland web site (
http://heilandelectronic.de/home/lang:en).
From this info I derive that the USB is acting like a serial port connection, it is not making a new disk on the connected PC from which the data can be copied (like a camera or external drive).
So custom made software must access the data via serial port, there is no file on disk which can be opened.
For now my strategy is to buy the USB option without the DensePC software and see if I can write a routine to get the data. That would allow me to implement any calculations I can think of and automate the process instead of collecting and processing density values by hand.