Yes there are darning plates made for the 15-90. The trick is to finding an OEM Singer unit.
They look like this:
[ATTACH=CONFIG]488986[/ATTACH]
Sew-Classic has generic feed dog cover plates and darning feet of several designs. I have both sizes of the Singer OEM ones and several darning feet, but I haven't used them. So, I can't say which works best on what machine.
Joe