Most of this comes from RoD Half-Elf Substitution levels or AU Bardic Sage. Lawful Alignment, Lay Down the Law, and some of the tweaks to Bardic Knowledge are mine.

Law Master Bard
In some pre-literate societies, the conflicts and legal proceedings were mediated with the aid of highly trained individuals who had committed the entirety of their people’s legal code and legal rulings to memory. The specialized memorization training these individuals received could also be used for memorizing songs and epic poems and they would often serve as an entertainer in the great hall when their legal services were not needed.
Alignment: Any Lawful
Base Save Bonus: The law master bard has good will saves, but has poor Fortitude and Reflex saves.
The law master bard has all the standard bard class features, except as noted below.
A law master bard learns and casts spells as a normal bard, with a few exceptions. In addition to the normal number of spells known, a law master bard knows one divination spell of each spell level he is capable of casting. For example, a 1st-level law master bard knows four 0-level bard spells plus one 0-level bard spell of the divination school (such as detect magic, know direction, or read magic. The law master bard's number of spells per day does not change.
To learn or cast a spell, a law master bard must have an Intelligence score (not Charisma score) equal to at least 10 + the spell level. All other spellcasting factors, including bonus spells and save DCs, are still determined using the law master bard's Charisma score. Add the following spells to the law master bard's class spell list: 1st—detect chaos/evil/good/law; 2nd—zone of truth; 3rd—arcane sight; 4th—analyze dweomer (lowered from 6th), sending; 5th—contact other plane, greater scrying (lowered from 6th); 6th—true seeing, vision.
Bardic Knowledge
A law master bard gains a +1 bonus on all bardic knowledge checks, +5 if it relates to laws or customs.
Bardic Music
A law master bard's powers of inspiration are not as persistent as those of a traditional bard. His ability to inspire courage, inspire greatness, or inspire heroics lasts only 3 rounds after the ally can no longer hear the law master bard sing, rather than the normal 5 rounds.
Command (Sp):
A 6th-level law master bard with at least 9 ranks in Diplomacy may spend one daily use of her bardic music to issue a command (as the spell) to a creature within 30 feet. Alternatively, the command can affect all creatures that the bard has affected with her soothing voice or fascinate ability. A Will saving throw (DC 11 + bard’s Cha modifier) negates the effect.
This benefit replaces the standard bard’s ability of suggestion.
Secrets of the Diplomat:
At 8th level, a law master bard adds the following spells to her spells known: 1st—command; 2nd—zone of truth; 4th—sending.
This benefit replaces the 4th-level spell learned by a standard bard at 8th level. From now on, the bard knows one fewer 4th-level bard spell than normal (not counting the spells she learned from this substitution feature).
Lay Down the Law (Sp):
A 18th-level half-elf bard with at least 21 ranks in Diplomacy may spend one daily use of her bardic music to issue a task to one or more fascinated subject. The law master choses whether to apply a geas to a single fascinated creature or a lesser geas to any number of fascinated creatures.
This benefit replaces the standard bard’s ability of mass suggestion.

Note: While it could thematically be argued that these bards should be illiterate and loose decipher script, however many preliterate societies had limited-use proto-writing systems known to a select few; a law master would likely know these systems and use them as memorization aids. This would allow them to quickly become literate upon exposure to full writing systems. An adventuring Law master almost certainly has had this exposure.