Adding my own, few results. None of these is particularly good, and I’ll probably just stick with artificer 5th if I don’t find something better.
Keeper of the Cerulean Sign (Lords of Madness)
Requires Craft Wondrous Item, obviously trivial for an artificer, and Aberration Banemagic, which is garbage, especially for an artificer (I guess it would apply to chill/heat metal? That’s about it for infusions that can directly damage an aberration, though...). And some skills, most of which aren’t artificer class skills but which an appropriate dip could mostly handle.
And at 1st, it gets detect aberrant taint as a supernatural, at-will ability. It’s OK, I guess, in a themed campaign. Not worth the feat burned on Aberration Banemagic though.
Moonstar Agent (City of Splendors: Waterdeep)
Requires not one but two of those awful +2-to-two-skills feats, in Investigator and Stealthy, and otherwise requires identify and disguise self. Artificer handles identify, of course, but disguise self will take finagling, or limit you to rather particular dips for your 1 other level. You also need bardic knowledge, lore, or similar, so really we’re talking about a cloistered cleric dip with the Trickery domain.
But if you do all that, you get (drumroll, please...) Skill Focus (any Knowledge). Hurrah.
Nosomatic Chirurgeon (Dragonmarked)
Second only to champion of Gwynharwyf in difficulty to spell, the nosomatic chirurgeon is for House Jorasco healers. You need the Mark of Healing, as well as ranks in Heal and Knowledge (nature)—that is, non-artificer skills, though Knowledge (nature) requires only 4 ranks.
And you get +1 to the caster level of your Mark of Healing spell-like abilities, +1 to saves against disease, and +1 to Heal checks. And the ability to spontaneously convert spells into the inflict spell of the same level. All-in-all, not good.
Paragnostic Apostle (Complete Champion)
I rather like this prestige class. Just sayin’.
Problem is requirements: “Ability to cast 3rd-level spells or use spell-like abilities of equivalent level.” Obviously, by definition, 3rd-level spells are out, but 3rd-level spell-like abilities are much more easily obtained. If nothing else, a dip in dragonfire adept allows you to select endure exposure as a least invocation that counts as a 3rd-level spell-like ability. There may also be racial options here.
Anyway, what you get is... decent. It stacks with other classes for turn/rebuke undead, but since it explicitly doesn’t grant those abilities, probably not useful to us. It also grants lore, and its levels count double for the check, which is nice. And then you get your choice from a fairly-lengthy list of options, most of which are +1 to spell DC or caster level in certain situations. Nice, though nothing amazing.
Sentinel of Bharrai (Book of Exalted Deeds)
Requires Sacred Vow—so you already know it’s going to be bad—and Vow of Obedience. I don’t remember exactly which one that was, but the Vows are all pretty bad. Also requires 8 ranks in Knowledge (nature) (along with Arcana and Spellcraft), which is a minor problem for an artificer.
Anyway, you get the ability to make your energy-damage spells do nonlethal damage to plants and animals—I’m not sure any infusions are at all likely to benefit from that. And you get Track as a bonus feat.
As you might expect, requires bardic music, so almost-certainly combined with a dip in bard, though I guess you could use Augustinius’ Folly, a “slightly-cursed” magic armor from Dragon vol. 324 that grants “inspire courage once per day as an 8th-level bard.” Would be difficult to justify coming across by ECL 5th, though.
Other requirements aren’t much better, with a ton required and many of them not being class skills for artificers. The 8 ranks in Perform are the most problematic, though; the other cross-class skills require 4 ranks each, making it easier to cover them with a dip, and the other 8-rank requirements are in skills that are in-class for the artificer. As a high-Int class, probably do-able. But it gets worse, because we require Magical Aptitude—another +2-to-two-skills feat, our favorite—and Storm Magic—+1 CL in a storm, which isn’t entirely awful, but is pretty niche. In all, pricey entry.
In addition to progressing our infusions, stormsinger 1st also gets another daily use of bardic music, and a bardic music effect to use gust of wind as a spell-like ability. Hard to imagine many days go by where you’d prefer to do that over inspire courage, at least with only two daily uses of bardic music.
Thrall to Demogorgon (Book of Vile Darkness)
Requires BAB +4 (so artificer 4th and a full-BAB dip) along with 2 ranks in each of a few knowledges (arcana, religion, any one other), and the Willing Deformity and Thrall to Demon feats. You also have to be Chaotic Evil and perform a ritual sacrifice, natch.
1st level grants, along with spellcasting, proficiency with all simple and martial weapons, and all types of armor and shields. Which is, actually quite good. Probably not worth the feats; certainly not worth dealing with Demogorgon, and dragonslayer does the same thing with better other features and more conveniently for most gishes who care about those proficiencies, but ya know, in a vacuum, good. Oh, and also +1 natural armor bonus to AC, which is yawn.