If your board has an exposed debug circuit, you should go with a JTAG/SWD based flash programmer. As you say, there's several choices, and I suspect sufficiently portable. The salesperson has to connect a cable either way. Add a keyed socket on the board side to avoid trouble.
What I want is a unit which can be attached to the serial port of my board and used to program the processor. This device would contain it's own processor and memory and operate independently not requiring a PC except to load the hex file to be programmed.
Yes I have Googled half the day with nothing to show for my effort. There are any number of AVR devices but alas nothing I saw would work.
There are several stand-alone JTAG programmers but I need to put these in the hands of salesmen and I would expect problems.
Please be more specific. Standalone can be interpreted. For example, if your board had a debug circuit, I would consider the LPCXpresso debug utility program standalone, since it can program flash independent of the LPCXpresso IDE. If you're looking for an unhosted solution, I'm not familiar with any. Such a programmer would at least require a serial port. A quick Google search revealed nothing which meets your requirements.
Thanks and regards,