Toki Pona
it would be a literal crime if I included Esperanto here without including my favorite conlang. Toki Pona's numbering system is next to non existent, so it can be adapted into seximal without changing anything at all.
0: ala
1: wan
2: tu
3+: mute
"all": ale
this system isn't very useful, so Tokiponists extended it into an almost numbering system, using "luka" (hand) to mean five, specifying that "mute" always means twenty, and that "ale" (sometimes ali, to avoid confusion with ala) always means a hundred. the seximal equivalent would be something like this.
0: ala
1: wan
2: tu
3: tu wan
4: tu tu
5: tu tu wan
10: luka
11: luka wan
12: luka tu
13: luka tu wan
14: luka tu tu
15: luka tu tu wan
20: luka luka
30: luka luka luka
40: luka luka luka luka
50: luka luka luka luka luka
100: mute
200: mute mute
300: mute mute mute
400: mute mute mute mute
500: mute mute mute mute mute
1000: ale
2000: ale ale
3000: ale ale ale
4000: ale ale ale ale
5000: ale ale ale ale ale
it would very much be against the spirit of Toki Pona to have a way to say "unexian" other than "ale ale ale ale ale ale". that workaround you came up with just now? nope, can't do that. it's not that it doesn't work, it's just that the whole point of Toki Pona is the restrictions in its vocabulary.