Rubato

It doesn't have to be made up completely of diatonic chords to keep its tonal center, tonality isn't that fragile, especially in this case where we have a Parallel key.

If you take this progession



and elaborate it with secondary dominants



it changes nothing in the function of the initial triads.

SIRIUSB

Secondary Dominants function as passing chords, just like any notes other than A B C# D E F# G# would function as passing or lead tones in the key of A Major.

Rubato

They still don't effect the tonal center. Calling them passing chords doesn't do them much justice since not many chords can be said to have structural significance, the first two bars are merely a prolongation of I-IV and bars 4-7 a prolongation of V, so really "passing chord" can apply to almost every chord in a piece of music.