MPlayer has an mcdeint filter which takes one field per frame as input and uses motion compensation to fill in the details, as it were. If this filter was ported to avisynth, you could try it.
mcdeint=[mode[:parity[:qp]]]
Motion compensating deinterlacer. It needs one field per frame as input and must thus
be used together with tfields=1 or yadif=1/3 or equivalent.
<mode>
0: fast
1: medium
2: slow, iterative motion estimation
3: extra slow, like 2 plus multiple reference frames
<parity>
0 or 1 selects which field to use (note: no autodetection yet!).
<qp>
Higher values should result in a smoother motion vector field but less optimal
individual vectors.
Although it looks like I may have misunderstood your question. If you're given 25fps and you want to restore 50 fields per second interlaced, or 50 frames per second progressive, you will need some sort of temporal frame interpolation filter. I'm not sure know how to do that.