It depends on what you want to achieve.

Most likely she will not break up with you because of such thing and it will go pretty well.

Keep in mind that women do not give shit in what situation they are as long as they get attention from the guy they like. all what matters is that you see HER in those situations and not some other girl. If she is not totally frigid ice queen she will get into any of your fantasy and enjoy it. Just make sure she understand that you are not going to cut her throat when she is asleep.

I would recommend talking about that in the situation when she is on the high libido mood. In those situation people do not give a shit about what is happening they will accept anything. On the other hand id she is frigid she may freak out because she is already faking orgasms and she is not willing to get into more weird things

she may also refuse and most likely she will refuse at first but it will not have any bad consequences. very likely she will change her mind withing few days if you presented everything properly with high optimism and enthusiasm telling her how great it is.