Monk Multiclassing
If your group actually enforces the monk multiclassing restriction, and you want to go back to Monk for some reason, Monastic Training from Eberron Campaign Setting allows you to multiclass monk with any one other class. The only good use of this feat is Tashalatora (see Psionics below), since it’s very rare for anyone to leave monk and then want to get back into it. Many players consider two levels of monk to be the absolute maximum anyway.
Arcane Spellcasting
There are no feats that allow arcane spellcasting levels, of any type, to stack with monk levels for the purposes of flurry of blows or unarmed strike damage, which are what you really want. Mixing arcane magic and monk features usually only happens by using prestige classes, such as the enlightened fist (Complete Arcane).
Various gish-related feats work for monks as well as anyone else. Arcane Strike (Complete Warrior) is OK.
Intelligence-based Casting
You can take Carmendine Monk (Knights of Valor) or Kung-fu Genius (Dragon vol. 319) to change the Wisdom to AC bonus to Intelligence to AC. Carmendine Monk also counts your monk level as 2 higher for the purposes of one of the following (chosen daily): unarmed strike damage, unarmored AC bonus, or fast movement. Neither’s particularly good, but they exist.
By the way, the Carmendine Monk feat doesn’t actually require Wizard at all; you can just use it to make an Int-based Monk. For a monk/wizard, though, it’s what you've got, since it’s strictly better than Kung-fu Genius.
Charisma-based Casting
Ascetic Mage switches Wisdom to AC to Charisma to AC, and lets you stack sorcerer and monk levels for the AC bonus. It also lets you trade spell slots for an attack/damage bonus to unarmed strikes equal to the spell level, as a swift action.
Notably, the Wis-to-AC swap to Cha-to-AC isn’t actually specific to the monk class—any source of Wis-to-AC can be changed to Cha-to-AC this way. This could be good for swordsages (Tome of Battle), moon-warded rangers (Dragon vol. 340), and saints (Book of Exalted Deeds), among others. This is the only effect that actually matters in the feat, too—the level-stacking thing only applies to the extremely slow-growing AC bonus, and the swift-action spell-sacrifice thing is just not worth the spell slot, or the action for that matter.
Invocations
There are no feats that directly stack invocations with monk class features. The enlightened fist prestige class, at least, does work for invokers, as it only requires an arcane caster level and not actual spellcasting.
Precocious Apprentice (Complete Arcane) can arguably get you a 2nd-level spell as a dragonfire adept or warlock. Good luck finding a table that will allow it. If you do, you can then take Ascetic Mage, and enjoy Cha-to-AC.
The Eldritch Claws feat (Dragon vol. 358) gives a warlock a pair of claws that deal damage equal to his eldritch blast damage plus his unarmed strike damage. Obviously, this is better if your unarmed strike damage is good. The Beast Strike feat (Dragon vol. 355) allows your unarmed strikes to deal damage equal to your normal unarmed strike damage plus your claw (or slam) damage. Combine the two to deal unarmed strike damage + unarmed strike damage + eldritch blast damage as an unarmed strike attack. I.e. you can flurry it. This combination is not particularly appreciated by most tables, however.
There are better options than monk for progressing one’s unarmed strike damage, though, like swordsage or Shou disciple.
Psionics
You did not mention psionics, but I want to point them out because they have a much better option for monks than do any other sort of magic.
Tashalatora from Secrets of Sarlona allows you to choose any one psionic class as progressing your monk class features, and can be taken as a monk bonus feat at monk 2nd. It requires Monastic Training (Eberron Campaign Setting) in the desired psionic class, which itself can be taken as a monk bonus feat at monk 1st.
Thus, a 2nd-level monk/18th-level psychic warrior or 2nd-level monk/18th-level ardent (Complete Psionics) becomes a very solid build for an unarmed mystic combatant (both classes are ¾ BAB and depend on Wisdom). Practiced Manifester (Complete Psionics) is a must at 3rd level, especially for ardents, though.
However, strictly speaking, neither Monastic Training nor Tashalatora explicitly requires monk levels. Which means you can be a human psychic warrior or human ardent who has full unarmed strike progression, flurry of blows progression, and AC bonus progression of a Monk of the same level at every level of the game. This is even better, since it protects your Manifesting, which is way better than what two levels of monk offers, even if it does cost you a couple of feats (one of which you basically “get back” since you don’t need Practiced Manifester). You should clear this with your DM first, though, since some take a dim view of monk-free Tashalatora.
