Inadvertently turned my character - on my first playthrough - into someone with a bit of both and now he's just not very good at much. He's leaning more toward tech but didn't really take a specialization like thrown or firearms.

Has anyone made this work, keeping that balance right in the middle yet still having a viable character that can finish the game? Beyond making a diplomacy character, that is.

It's not a good idea. You ought to consider your three options as Magick, Tech, or neither. Balance between both is largely self-defeating.

That said, I believe it can be done, but it requires planning by an experienced player with an extensive knowledge of the game.

There are ways to reset and reallocate your skill points if you don't want to start over.

With a neutral character, most spells and pretty much any non-neutral equipment will only work at half power at best, so spending too many points on spells and schematics only leaves you underpowered. If you really want a mix, it's better to also develop something on the neutral side that would help. For example, a fighter who knows some low-level buff spell (which work just fine at neutral aptitude) and has just enough Intelligence and Constitution to maintain it efficiently in combat can use the points that would otherwise go into the relevant stat to learn some low-level schematics. Can't really imagine a non-combat magitek mix, unless it's a charismatic bastard with an army of followers (whatever offensive spells you might be using won't be too powerful at neutral aptitude).

But first playthrough is still better with a neutral character, IMO. Once you know what kind of stuff you can obtain in the game, it's easier to plan ahead and create a more complex build.