In keeping with the season, I activated my 630m transverter and achieved amazing ranges on 474kHz. Despite an unsuitable under-roof antenna, but of course only in Wsprnet.
Now I want to plan an activation at 630m again. Certainly there are hardly any chasers at 630m who could answer me on this wave. But maybe they would decode me in, for example, FT8 and then respond on shortwave.
As long as both you and your QSO partner’s licence allow you to use the bands in question then cross band is legal for SOTA. There are many who have had satellite QSOs on say 2m up 70cm down or 70cm up 2m down. Or QO-100 13cm up 3cm down.
An example of what is not legal… UK hams can have cross band QSOs with hams in the UK on ham frequencies or non-UK hams on other licenced bands. UK hams could hear EI hams operating on 40MHz and have a cross band QSO on 28MHz or 7Mhz etc. 40MHz is an EI ham band. UK hams can get an I&T licence and also transmit on 40MHz. But the I&T licencing forbids having QSOs so if I hear a UK ham sending under I&T licence on 40MHz I cannot have a cross band QSO with him.
So yes, activator on 630m and chasers on any licenced ham band is 100% OK.