Some possible alternatives have been listed in the recently published paper under the section "Industrial catalysis aspects", although in the preceding section it is pointed out that stabilizers often employed in commercial catalysts to decrease the loss of K may impede RM formation.
K–Mn synthetic zeolites have also been shown a few years ago to emit Rydberg K upon heating: (PDF) Emission of highly excited electronic states of potassium from cryptomelane nanorods (researchgate.net)
As well as some alkali-doped transition metal carbides and nitrides materials, several years ago: (PDF) Potassium at catalytic surfaces - Stability, electronic promotion and excitation (researchgate.net)
Potassium apparently has to be bonded in a special way to get desorbed in a Rydberg state. This was mentioned for example in https://doi.org/10.1039/B817380J (2009) by Kotarba and Holmlid, citing this article: https://doi.org/10.1021/ja0542901