If you're looking for a way to do this without dropping anything, consider having a wand chamber installed in your bow (I checked, dungeonscape doesn't specify melee weapon). Then get a metamagic wand grip and attach it to your wand of arrowgrip, and stick the entire thing into your bow.
Then you can use, consuming extra charges to account for level differences, any 1 metamagic you know when you activate the wand, and you can activate the wand without actually getting it out- just by wielding the weapon.
Thus, three times per day you could maximize arrowsplit, or chain it to hit every arrow you use for your greater manyshot- or whatever.
Alternatively, (or perhaps as a supplement) you could pick up a caster's glove (MIC), which lets you store a magic item, including a wand, inside like a glove of storing, and activate it as though you were wielding it.
Unfortunately, caster's gloves are pretty bloody expensive (like 60,000 gp) and only come one glove at a time.
If you're not tied to your immediates, and since you're already using one swift action, you aren't, you could conceivably use both. I'm not seeing anything preventing you from casting arrowsplit on an arrow already hit with arrowsplit, so twinning and the like should be functional. The trick is to get both metamagic feats on the same casting (maximize and twinning) for optimal effectiveness, and then run it through the splitting on the bow.
The alternate, much cheesier (if pricier) option is to just get an adamantine bowstring of continual maximized arrowsplit made.
