I only looked at speeds available from autocomplete (17.1 btw), but it actually does appear to be a hidden option that passes commit check. I don't need it at this point now so I'll leave testing up to someone else... Thanks for the follow-up.
{master}[edit interfaces ae0] test@ex9200-re0# set aggregated-ether-options link-speed ? Possible completions: 100g Links are 100G 100m Links are 100M 10g Links are 10G 1g Links are 1G 25g Links are 25G 40g Links are 40G 50g Links are 50G 80g Links are 80G 8g Links are 8G oc192 Links are OC-192 {master}[edit interfaces ae0] test@ex9200-re0# set aggregated-ether-options link-speed mixed {master}[edit interfaces ae0] test@ex9200-re0# commit check re0: configuration check succeeds re1: configuration check succeeds {master}[edit interfaces ae0] test@ex9200-re0#