The only way that I know off to get an Aaddb9 chord is A/Bb. That will give you A-C#-E-Bb but probably not in the inversion that you want.
I hadn't thought of that, so that may be one way (with the snag you already spotted tough, but still), thanks for the idea.
What complicates the matter in this particular case is that my desired root note is neither the root note of the I-chord, nor of the 9th like your suggestion, but of its relative IV, as I mentioned.
Its a tricky situation indeed

p.s. Is it possible that a "dimsus" chord may be known under another name? I'm wondering if that alternative is in the pulldown and my limited chord knowledge just isn't recognising it as the same chord...grasping straws here, I know.