I am just pre-cooking my next batch of chicken using 50% mustard oil (the other 50% being rice bran oil).
OK, ate the resulting curry last night and it was Not Bad (tm). This was basically a KD1 Chicken Madras with 1/5 of the oil replaced by surplus lime pickle oil (i.e., the oil which preserves lime pickle, and of which there can sometimes be an excess in any given jar), and with my usual modifications of twice the volume of gravy and twice the quantity of spices per unit volume of gravy, no added garam masala or fresh tomato (in future I shall refer to this as KD1/PT, to save keep repeating my modifications !). The mustard oil flavour clearly came through in the final dish, but it was not overly intrusive as I had feared, so I have now pre-cooked the remains of the chicken in the same sauce as I used for the first batch.
On balance, is pre-cooking in mustard oil beneficial ? No. It is useful as a variation, and there may be some dishes (I am thinking in particular of dhansak) where that particular flavour might be complimentary, but for everyday BIR preparation I can see no benefit.
** Phil.