I believe the usual reason for not injecting bolus insulin into thighs is that using our legs affects the absorption of the insulin. I assume, as you are having a tummy operation, you will be resting so you should be able to use your thighs more.
I recently had a tummy operation and found, as long as I avoided the wound, I was able to continue using my tummy for insulin.
The other areas I have read about (but not used myself) are upper arms (aka bingo wings) and bottom. I understand, you should be able to use any fatty areas. Test it out before your op.
Remember to avoid injecting into scar tissue after your operation. This will affect the absorption of the insulin.
I hope your op goes ok and you are able to rest afterwards - I took up knitting and had a couple of weeks of NetFlix and knitting which resulted in a new jumper.