I found the Wintec WBT-201 for about 100 Euro/US-$ which seems to be very interesting. Compare e.g. here or (with more info) in German here. The question is, how data of the log can be used with exiftool.

So I have a feature request:

Syncing of gps-logs with exiftool:
There are different formats around, like NMEA, kmz (GoogleEarth), txt (PaPaGo), plt (OziExplorer), htm (Visual Earth), gpx (Single/multi-GPX, Touratech QV), etc. While I don't know these formats, I think it should not be very difficult to do the following:

Searching in the log for time and gps-data
definition of a time-difference between camera and gps-tracking-system
definition of the time-field which has to be used for syncing (with a default time-field)
tolerance if there was no exact match of the time
option: using the next or previous entry in the gps-log, if no time-match was found within the tolerance.
definition of the name of a default import-log, optional another one.