Sync everything to GPS. Record a harmonic of the carrier on the same tape, or just record the carrier itself on a parallel track. Have the SDR use this reference tone to undo any tape speed fluctuation artifacts. You can bring the deviation from DC so low that your comment no longer has any relevance to tape machines and just becomes a pedantic comment that mathematically perfect DC doesn't exist in physical reality.
A metaphor should be referring to something common, not something that can be made in theory to fit a different set of requirements, but does not really exist.
