Up to now there has not been a PPA providing this because of the complexities inherited from the old OOo build.
With the 3.5/precise release both the upstream build system and the packaging at Debian and Ubuntu have been cleared up enough to make this basically feasable -- if excluding l10n, binfilter and mozilla. Such a PPA would still likely burst default size limitations within one month and still hug a builder machine for >3 hours (on a fast builder) because there is no easy way to reuse a ccache in a ppa -- so I guess there is some clearance needed for this additional resource use.
Next question would be what to build as there are at least two/three active branches all the time. For example right now:
- the first being master which will be in the p+1/LibreOffice 3.6 release
- the second libreoffice-3-5 which will be in the precise/LibreOffice 3.5 release
- the third being libreoffice-3-4 which was in the oneiric/LibreOffice 3.4 release
Master is obviously the most interesting, but for QA the release branch (which will be in the next Ubuntu release) can also be interesting.
So even with this, there is a lot of details to work out (like versionnames to use etc.). If you are interested, I would be glad to help out though!