I'm trying to produce progressive MPEG4 output from mixed 720p, 1080i and pal-dv clips.

I suppose this should work when Scanning is set to force progressive in rendering project, or when the project is set for progressive target.

Is this correct?

Since this isn't working for me with kdenlive svn3376 and mlt svn1428, is there a way to force deinterlacing through the clip properties, kdenlive project file or the .westley temp file which I manged to save?

For reference, these are the clips used (ffmpeg -i):

Input #0, mpegts, from '00022.mts':
Duration: 00:00:21.35, start: 1.000067, bitrate: 15727 kb/s
Program 1
Stream #0.0[0x1011]: Video: h264, yuv420p, 1920x1080 [PAR 1:1 DAR 16:9], 50 tbr, 90k tbn, 50 tbc
Stream #0.1[0x1100]: Audio: ac3, 48000 Hz, stereo, s16, 256 kb/s

Input #0, avi, from '1.09-01-27_07-15.00.avi':
Duration: 00:00:16.40, start: 0.000000, bitrate: 30585 kb/s
Stream #0.0: Video: dvvideo, yuv420p, 720x576, 25 tbr, 25 tbn, 25 tbc
Stream #0.1: Audio: pcm_s16le, 48000 Hz, stereo, s16, 1536 kb/s

Input #0, dv, from '002.dv':
Duration: 00:00:14.40, start: 0.000000, bitrate: 28800 kb/s
Stream #0.0: Video: dvvideo, yuv420p, 720x576, 28800 kb/s, PAR 59:54 DAR 295:216, 25 tbr, 25 tbn, 25 tbc
Stream #0.1: Audio: pcm_s16le, 32000 Hz, stereo, s16, 1024 kb/s
Stream #0.2: Audio: pcm_s16le, 32000 Hz, stereo, s16, 1024 kb/s

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'p1000951.mov':
Duration: 00:00:03.50, start: 0.000000, bitrate: 25356 kb/s
Stream #0.0(eng): Video: mjpeg, yuvj420p, 1280x720, 30 tbr, 30 tbn, 30 tbc
Stream #0.1(eng): Audio: pcm_s16be, 16000 Hz, mono, s16, 256 kb/s

The dvvideo streams I can deinterlace when playing with "mplayer -vf pp=fd file", but mplayer refuses to play the .mts file though line scan stripes are visible with ffplay and inigo. But how about kdenlive rendering backend...

I tried upgrading ffmpeg to latest git version but that didn't help.
Adding ffmpeg option "deinterlace" or "-deinterlace" to the rendering profiles did not help either. I can see "progressive=1" being sent to inigo, so I guess this a bug in inigo/mlt?

As far as I (MLT developer) believe at the moment - will require analysis to confirm - the deinterlace will work as you expect. However, if MLT is unable to determine that the source clip is interlaced, it does not. Also, the deinterlace routine is not so good, so sometime it might still look interlaced.