Is it possible to use SNP heterozygosity as a proxy for Indel heterozygosity?
SNPs, insertions, and deletions are generated by different biological processes (see for example here). I would expect the per-generation rates of these mutational processes to vary across species. I would also expect the degree to which each mutational process varies across species to vary as well. I think the answer to your question is "probably not without further information".

However, priors are a feature of Bayesian statistics. In Bayesian inference, it is good practice to redo one's analysis using several priors to establish how sensitive one's results are to the chosen prior. I would suggest that you try to call indels using a range of priors and compare the quality of your call sets to determine how important the choice of prior actually is.