I'm taking the DipABRSM this coming season (Oct-Dec 2009). Repertoire-wise, this is what I think:
1. the guitar syllabus has many small pieces (say "any 2 Walton's Bagatelles" instead of a complete sonata) due to the nature of guitar literature itself;
2. consequently, I think, we guitarists need to play at least seven pieces (5m each) from at least 4 composers to make up the 35m time-requirement, and this is easily be more (I play 10 pieces from 7 composers);
3. that means, our performance will be a generalist one, more like a CD of 'the best of classical guitar' than of "baroque and classical guitar";
4. somehow, that will effect the viva-voce. Instead of thinking about 2 pieces of sonata, we must dig up more pieces from more composers. The good side of this is it becomes easier for us to write those 1100-word programme;
5. I would personally say that one plays as much as pieces that he's played for sometimes, and learns as few as possible new pieces. The point of an exam, I think, is just to get standarized. So if the requirement is 5, let's just do it 6. We'd better concentrate on passing the exam than on taking a chance with showing off our skills.
These are another crucial aspects for us guitarists:
1. we tend to re-tune the strings after a piece. I personally use a tuner and really practice it! This issue is especially real if we must tune 6=D to 6=E, or worse, you play Koyunbaba

Now say you need 15 seconds to tune, play 10 pieces, that is already 150-second or 2.5-minutes extra. Right? So think and practice this tuning issue too;
2. the sitting! Do find out what kind of sitting is most convenient for you. When I played the first-piece for grade-8, I remember cursing the chair as I played. That was because I just sat and played without testing the comfortableness of my sitting. As a result, everything felt wrong and uncomfortable [luckily I played a piece that I'd played for ages

]. So, try to find out what comfortable sitting is for you personally, and do try to get it during the exam. Make sure you know how to adjust the piano chair, or else, just bring your own plastic chair;
3. page-turning! The benefit of reading is to help us avoid mind-excursion and also eye-contact!
You know, practically it is only us guitarist who must look at the audience during a performance. Due to their sitting, pianists have no direct eye-contact with the audience; violinists too, who can just stand and move around their body and feet; cellists normally are facing their accompanist, right? But not us, because we do have a fixed position facing the audience. So, reading can help us to avoid that eye-contact. Imagine you catch a nasty critical look on the examiner face as you're playing, then you start to worry, then you start to slip. So, reading in this exam setting, I think, can work well for us.
For the page-turning, I scan the music, then cut-&-paste the bars as such that I can turn the pages at my convenient time. Sure I also practice this page-turning.
Well ... I wish this is useful for you!