Je voudrais remercier tous ceux qui ont aidé avec l'organisation de l'événement.
How is tous pronounced here? I know that the pronunciation depends on whether tous is a pronoun or an adjective, but I'm not sure what it is here.
Here, tous is an adjective agreeing with the pronoun ceux so according to the rule you linked to is pronounced /tu/.
Had it been pronounced /tus/, that would have lead to /tus.sø/.
Je veux les miens plus ceux de mon frère (I want mine and those of my brother)
where plus ceux is pronounced /plys.sø/ and:
Je ne vois plus ceux de mon frère (I don't see those of my brother anymore)
where plus ceux is pronounced /ply.sø/.
It's always pronounced /tusø/.
Tous is a determiner here (hence feminine form toutes celles /tut sɛl/), but the /s/ is combined to the one at the beginning of ceux, so it would be pronounced the exact same way whether tous were /tus/ or just /tu/.
The only case I can think of where the pronunciation may be different is in Quebec, for speakers who have only the invariable form /tut/ regardless of what standard grammar would have. In that case, you would have /tut sø/ instead (spelled toutes ceux, but toutes ceusse is more likely when you get to that degree of joual), but this is considered nonstandard even for Quebec French.