Thanks for moving your request here @ben_levine. It’s more likely to be noticed when in the correct thread.
It may not be exactly what you’re asking for, but I’d like to just mention two things that have been noted before:
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The 24 hour limit is per-item. You can download as many albums, playlists, radios etc as you want, but each one is limited to 24 hours currently for performance reasons in the current implementation
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A neat workaround is to use smart playlists with a filter to restrict to stuff you haven’t heard recently. The effect of this is the playlists become sliding snapshots of fresh music that will refresh from the server periodically as you listen to them. It’s almost like having your whole library with you.
As I say, it’s not the answer to the question you asked, but it may be a workable solution for you depending on your use case, at least until/if Plexamp gains longer limits for downloads 
