Hi Paul.
I believe historically this has been acceptable.
The first amateur signal to cross the Atlantic on 136kHz was on 10th Sept 2000. That was high power at the EU end with an antenna about 200ft high and resulted in a crossband QSO using QRSS 3 on 136k and normal CW on 20m in Canada.
Following that, if my memory is correct there was a gap of around a few weeks followed by some QSOs that did take more than one night. One of these may have been the first 2-way 136k QSO. G3LDO from memory.
Mike XDV or Alan will probably recall the usual suspects of the time.
On the other hand, as we were reminded by Mal a few days ago, in the winter months DFCW QRSS or even JT9 will not be a problem. It's encouraging to see your signal stable for hours at a time on the various plots.
I'm planning another remote operation from October 18 through the winter months on 136k and 630m. Just need to collect some antenna hardware and high voltage band switching parts over the summer.