If the culture is as informal as you say, then there should be no (big) problem on asking to schedule an appraisal interview sooner then the standard 12 months. In the comments on your question you describe that one of your main concern you want to discuss it that you want to work flexible working hours. I'd say that this is something that is typically discussed during appraisal interviews, so you should be within you rights to put it on the table. It might be a good idea to make clear that this is one reason you want the interview, if not the primary reason, with the person you're going to do the interview with. Also, if you achieve the flexible hours you want, I'd have this put on paper as some kind of addendum to your contract, so that it isn't brushed away later. You say that you have contributed and made your mark (in a way) I think this should give you some leverage.
On the other hand, if company policy doesn't allow flexible working hours in general, you might have been given an empty promise last year. It might be that your manager wasn't within his rights to decide this for the company, and that HR turned it down or that his boss didn't like the idea. If that is the case, you might not get it this time either. It might be a good idea to find out the exact reason why you didn't get what you were promised last year. The only way to find out if you can get it this time is to put it on the table and to make it clear that it is something that matters to you. I suspect it might cost you a bit of salary to attain those flexible hours. (Because people with flexible hours are sometimes seen as less dependable or more costly) In the end you're only going to achieve it if you make it clear that is matters to you. It might be a good idea to think through what would be an acceptable bargain for you before you ask for the interview. It will only be confrontational if you take the conversation down that road. The question itself isn't offensive in my opinion.