lfs mirror create|extend should support mirror flags setting when creating new mirrors so that all components belong to the new mirrors will have the flags set. The flags are not inheritable so it will only apply to the mirrors in current '-N' option. For example:
lfs mirror create -N2 --pool flash --flags preferred -N4 --pool hdd file
the first two mirrors on flash pool will have 'preferred' flag set and it won't be applied to the last 4 mirrors on the 'hdd' pool. Similarly, the mirror flags should also work for lfs mirror extend and be applied to newly merged mirror from victim file or specified by setstripe options.
Multiple flags are split by comma when existing. For example:
lfs mirror create -N --flags preferred,stale -c 1 -N2 -c 2 file
the first mirror will have both 'preferred' and 'stale' flags set, though I don't know why someone would like to set these two flags at the same time.