A couple of things I would like to note: 1. The Marshal has Knowledge as a class skill. Not Knowledge (All. Taken seperately). Just Knowledge. Now personally, I think that it works like other classes with all Knowledges, but certain people may choose to interpret it as 'all knowledges in one skill', which is dumb, but I dunno.
2. Motivate Charisma is rediculous on a diplomancer. Or any Marshal, really. Double my charisma bonus to charisma based checks? Hell yeah! Sign me up. A half-elf marshal with Motivate Charisma, a high charisma mod, and whatever that half-elf specific feat from Races of Destiny is that lets you roll twice for Gather Information and Diplomacy, would make an obscene diplomancer.
3. Don't blanket rule out half-orcs. The Desert Half Orc from Unearthed Arcana does not have a charisma hit, and from there you could pick up Leadership, Might Makes Right, Extra Followers, and take 5 (or less, if you want) levels of Orchish Warlord to have an ASSLOAD of followers and a fairly decent army. In addition, there are quite a few sources of Intimidate bonuses for half-orcs. Take the first level of the Half-Orc Paladin substitution level to get Monstrous Mein, 3 levels of Half-Orc Paladin for Charisma synergy and more intimidate, then go marshal and take Motivate Charisma.