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I think normal convention is to use sharps for all notes/chords based (rooted) on natural (A>G) and sharp scales (A#>G#) and flats on notes/chords based on flat scales (Ab>Gb). That suggests the key of the 'too' scale controls the construction of the transposed chord. In the case of a 'keyless' 'from', the construction of the chord suggests the key type. With a keyless 'too', I suppose that would depend on the user with the restriction of 'one or the other' for the complete tune.

A mechanical approach that may work is to make two circles of the chromatic scale spacing each note, separating the sharps and flats and natural notes equidistant (17 notes). Make one circle smaller to fit inside the larger one. The smaller one will rotate inside the larger. The outer is the 'from', the inner is the 'too'. This will result in a straight one to one relationship.
G to A transposition---G>A, G#>A#, Ab>Bb, A>B, A#>C, Bb>C#, etc.