Comment (by Roland Haas):

Turns out that simfactory already supported something very similar via its used of @ENV(FOO)@ syntax in option lists. See https://bitbucket.org/simfactory/simfactory2/commits/8eae826ee6372ed6b3d44b3f4428e06969be8185#Lmdb/optionlists/sciama.cfgT12

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2312/expand-env-variables-in-rhs-of-option-list