If you want to use an IDistort instead of the STMap you will have to use a colour expression node to modify the UV data slightly because IDistort pulls pixels from a position relative to the output pixel (i.e. 5 pixels from the left of the target pixel) whereas STMap uses absolute coordinates that are normalized (0,0 being the lower left and 1,1 the upper right corner of the image respectively).
Copy the UVs into the main stream of your texture and drop the following into a color expression node to adjust the UV data:
u*width-x
v*height-y
I'm using "u" and "v" in this case because I copied the data into the u and v channels. If your data lives in other channels you will have to use the respective channel names.
With the modified UV data you can append an IDistort node, set it to the correct channels and it should give you the same results as the STMap (the filtering will be slightly different though).
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