The thing that seems weird to me about the OP's situation is that I always thought you are hiring the person, not the technique. It's fine and dandy to suggest a way to play something, but there'd have to be a lot of cash involved for me to swallow my best judgement about how something should be played.
Hmm. I never thought of it that way. I mean, as someone who's worked on recording sessions and has seen the engineer suggest many different techniques for musicians (in order to get "that" sound), I feel like it never hurts to make suggestions if you have a very specific idea in mind for a certain sound or a specific song. But like I said in my original post, when it comes to changing the style or approach for an entire project, I think those sorts of suggestions tend to read a little differently.
Not to mention, I've been listening again to the recorded bass lines he gave me to work with and while I can hear a pick being useful and sounding "right" for a couple of them, all I really hear is just his technique which- I hate to say it- sounds kinda sloppy and noisy...something that I can't necessarily- and shouldn't have to- match. To me, honestly? It sounds like a guitarist playing guitar parts on a bass. Does it work? Sure, to a degree...I mean, look at Peter Hook! But that's just how I hear it.
So, pick or not, my interpretation of that style is going to be different than the guitarist's...because even though I started out playing guitar before picking up a bass, I've approached bass from a very different mentality, as something totally new and different from guitar. Also, I don't really identify as closely with the bassists whose styles he's referencing: Mike Dirnt, Matt Sharp (I think that's him? the bassist from Weezer), that sort of punk school of thought where it tends to sound a little noisier. I've just never used their styles as reference points for my own, until now.
So whether or not I use a pick, if he's asking me to cop his style completely, it will be a bit of a departure from what I'm used to. Which is fine, but like I said, I'm still figuring out how to do that while still preserving my own "feel" for these bass parts. I guess for now I'm trying to follow the rules and play it his way, before breaking them and putting more of my own spin on it.