I asked a similar question a short time ago in IRC and the list of allowed addons are on the Rules page, whether that will change in the future however is down to the powers that be. I have no idea how much work would be involved getting them to sync in well with the server and it may be felt that time is better spent on other things right now.
On a side-note (and apologies for somewhat hijacking this thread with an extra question), from my old FFXI days I remember the Recast addon not just showing the countdown before spells/abilities can be used, but also it gave a countdown as to how long status' on yourself, positive and negative, would remain (from Protect, Haste etc to Weakness & Stat-downs). Is there anyway to get that included on Recast, or was that actually a different addon (for the life of me I honestly cannot remember!).
Also is there a mod file where we can reposition the display of ability/spell countdown? For me the font is a little large and in the way in its current position. I can live with it if there's no adjustment possible, I'd just prefer it elsewhere
*edit! Another thing I remembered from the retail version, though again I cannot remember if this was down to the Recast addon or not, I promise I can count to 3 and even up to 4 sometimes, but occasionally battlefield distractions will result in me losing focus. Does Recast here show the number of shadows left on Utsusemi?