Hi,
Is it possible to save (and load) MetaGraphs graphs to common formats while maintaining the metadata?
Not at this time. The formats in GraphIO.jl were designed to work with AbstractSimpleGraph and nobody’s created anything else.
PRs gladly accepted, though 
If you haven’t implemented it yet, you could probably adapt this code I recently wrote:
https://github.com/simonschoelly/SimpleValueGraphs.jl/blob/master/src/export.jl
Nice. Thanks.