If I recall correctly those feats in Complete Adventurer - Ascetic this and that - not only count both classes levels in total for determining the development of things like unarmed strike damage and sneak attack (or smite attack, etc), but they also allow one to freely multiclass between the two classes. So just create an equivalent Warlock / Monk feat for the player to take.
Hmm, should their blast count both monk and warlock levels, or should their number and level of invocations known / usable count both monk and warlock levels for the purpose of the feat? The monk, if I recall correctly, had his unarmed damage (and sometimes either AC, speed, or stunning fist DC) increase as if his monk level were the total combination of monk and ______ (determined by feat) levels.
[Edit]
Here is, in short, Ascetic Mage from the CAdv:
Ascetic Mage
Pre-req: Impr Unarmed Strike, ability to spontaneously cast 2nd level spells.
Benefits: Levels in Sor and Monk stack for determining AC bonus, and the AC bonus can use Charisma instead of Wisdom. You may multiclass freely between Monk and Sorcerer, although you must still remain Lawful in order to continue advancing as a Monk.
Also, you can sacrifice one of your daily allotment of spells to add a bonus to your unarmed strike attack rolls and damage rolls for 1 round that is equal to the level of the spell slot used.
So, it seems that you gain the AC bonus as if both levels stacked, and you could multiclass freely between the two, and you gain an extra bonus in which a spell slot could be used to increase the power of the attack and damage of the unarmed strike.
Of course, the warlock is a different can of worms from the sorcerer. The sorcerer losing a spell slot is significant, but a warlock has no slots to use / lose. Also, a warlock focuses on attacking from afar, so allowing the foe close enough for unarmed combat could be dangerous. Perhaps the warlock gains a bonus invocation (does not count against the allotment he is allowed to learn / know) that allows him to place the monk's stunning fist attack into one of his eldritch blasts. Also, protection will need to be high, so perhaps both class levels count towards not only the AC bonus, but also the warlock's DR and fiendish resilience.
So we end up with :
Ascetic Warlock
Pre-req: Improved Unarmed Strike, Monk's (or wisdom based) AC bonus, and Eldritch Blast +3d6.
Benefits: Your levels as a monk and warlock stack for the purpose of determining your AC bonus - which can utilize Cha should you wish it. They also stack for the purpose of determining the DC of any warlock invocations used, as well as the Warlock's DR and Resilience. Lastly, you gain a bonus lesser invocation that allows you to place the monk's stunning attack into your eldritch blasts.
Note: You may freely multiclass between these two classes, but you must remain Lawful to continue advancing as a Monk and Evil (or Chaotic, although it is not much recommended) to advance as a Warlock.
This might seem a bit strong, but compared to some of the other ascetic feats it should fit in well. Warlock Fiendish Resilience is not gained until level 8 and only increases at levels 13 and 20.The DR is gained at 3rd level and so is already had at the time the feat is taken. It increases at warlock levels 7, 11, 15, and 19. I would suggest, incidentally, that the DR be changed from Cold Iron to Alchemical Silver when this feat is taken, to reflect the lawful nature of the character. The protection of the AC bonus, DR, and Resilience will be needed for the character to remain alive. (Recall, this feat does not increase their number of invocations known, their eldritch blast damage, etc, nor does it grant the various bonuses the monk gains. As such, the character will be having trouble keeping up even with the benefits gained from this feat unless they are only dipping into one class or the other - and the feat requires at least 5 levels of warlock.)
