Interlaced output erroneously identifies as progressive

[KP1533TM2]

Hi there!
Recently I've had to transfer some miniDV tapes to DVD and I had some problems.
Symptoms
I've transferred my tapes with 'dvgrab' utility into a bunch of .avi files. Then I put some of them into devede-ng, with deinterlacing disabled. When I burn the resulting .iso to DVD and play it in physical SD DVD player, it shows completely normal, fluid motion (50i) picture, like the one from the camera itself. But when I try to play it with, say, VLC with deinterlacing on, instead of fluid motion I observe jerkiness, as if field order was set incorrectly. VLC can't be the problem, because it plays other DVDs with deinterlacing on just fine and fluid.
The problem
As I've found through a bit of tinkering, apparently, when devede-ng uses ffmpeg to prepare .mpg movies, it forgets to properly set 'Scan order' flag (and field order) for the resulting video file. Quering resulting movie .mpg file with 'mediainfo --fullscan' showed that it has progressive scan order, but it really should be interlaced /w bottom field first (just like the source). Seems that physical DVD player doesn't care about that (and real TV shows images that are naturally interlaced), but it throws off software players such as VLC.
My very dirty and temporary solution
In order to quickly patch this just to get this particular job done, I've added a couple of switches in 'ffmpeg.py' file, 'convert_file' method:
self.command_var.append("-flags")
self.command_var.append("+ilme+ildct")

But I'd suggest that devede-ng should detect those flags from source media file, somehow.

Maybe I'll fix this properly myself sometime later and issue a pull request to you, but I won't make any promises.
Thank you, anyway :)