I am unsure whether this question fits in this mailing list or the USRP mailing list but it concerns stream tags so here goes..
I'd like to tune the USRP using a DSP offset without touching the LO frequency. I'd like to do this using stream tags since it's important that the re-tune is synchronous with the sample stream. Judging by  this appears to be possible using the following structure:
tag.key = pmt.intern('tx_command')
command = pmt.make_dict()
command = pmt.dict_add(command, pmt.intern('dsp_freq'), pmt.from_double(the_offset))
tag.value = command
// add tag to stream etc
It turns out that this does nothing unless the LO freq is also supplied:
command = pmt.dict_add(command, pmt.intern('lo_freq'), pmt.from_double(some_center_freq))
But this causes the (relatively slow) LO to be retuned rendering the speedy DSP retune worthless.. I've seen hints about setting the tune request policy but this appears to not be supported by the USRP sink as a tag.. is it necessary to create a custom USRP sink block to properly use DSP retuning?