There are a couple of ways that the C-axis can react, depending on a parameter. It can be shortest distance always, or using the sign (+/-) to decide direction. Since you said +270 would do the trick instead of -90, I assume that the sign dictates direction in your case. I will also assume that you would not want to do this all the time for all your jobs, as it takes longer, so I would not really want to touch your post.
Have you tried outputting a manual NC "passthrough" with C-270 as the value? The result will depend on your post, but it could do what you need, then when it reads C90, it would stay put because it's already there. But this passthrough thing has to be enabled in your post, and where it outputs will vary as well, you'd have to try.
Otherwise you could setup a manual nc "action" to do this as you want it, but it requires fiddling with the post.
Also, since this will most likely be a rare occurrence, you could simply edit the code manually, because it's probably one of these exceptions for this job.
