I understand that its a well known property of HHG to shrink and 'self clamp' as it dries, but imagine if you left it to dry properly over a few days or perhaps even a week or more before sanding back, that it would not shrink back more than anything else if used to fill pores. Then again I would also imagine the ambient RH would have a lot to do with that and like glue up, you would want to make sure the air remained at the lower end of the hygrometer at least until you got a few coats of finish on. That said I have never used HHG as a pore filler to know so its just conjecture...If the idea is to use only traditional 'natural' products, perhaps you should investigate the egg white slurry method...its age old and, it still works even today, but then the effects of RH remains an important consideration there also....bloody wood pores, why can't they fill them stink'in selves??
Cheers
Kim