50p in 25i timeline - best interlaced workflow with breakout box

I am recording in 50p UHD and have to deliver for broadcast - so 25i HD (or 50i as some might call it).
For that I am using a breakout box: UltraStudio Monitor 3G.
I was hoping it could convert my 50p footage in realtime to 25i when previewing the clips in the browser window of FCPx but unfortunately it does not.
If I stay with 50i in Blackmagic Desktop Video settings it plays back the 25i timeline just fine but it plays the clips in the browser only in 25p (and maybe converts it to PsF). 
If I switch to 50p it plays back the preview files fine but I cannot playback the timeline properly.
Although the monitor is connected via HDMI I tried the settings for "SDI output": "1080p are output as 1080p progressive or segmented." Didnt help.
Anyone working in Europe (pal-region) and has a workflow for it?
I guess I can work in a 50p timeline and copy it over when everything is almost done or convert the final output via compressor to 25i, but I am not sure if copying from 50 to 25 generates new problems as clips could be shifted by one frame. Any ideas?