Do big manufacturers usually allow this?
Let's say I want a Mazda3, with the 2.5L from the highest spec model, but with the options from a base model. I can't do this in the website builder, but would it be possible to do it if I asked via email/phone?
yes
It depends. Some manufacturers might allow it, some might not.
No. They only offer a few configurations because that's what's profitable. The more choices you have the more expensive it is to produce. Taking up valuable assembly line time and space.
bullshit, unless it's some ass backwards company production is all computerised and it doesn't make any difference provided all the parts already exist
Always depends if its possible for them to do. But yes, you can ask them via mail or phone.
being polite and asking people with a bit of clout works wonders
If you're willing to pay a little extra for a special order they CAN do it for you, but finding a dealer who will play ball is the hard part.
My first car was a special-order 1995 Chevy Monte Carlo, so they still do it.
they're hiding the 2.5 in the luxury specs because they are Apple tier Jews.
they know that everyone would get the 2.5 for whatever the price increase is over the 2.0. you are paying the Xenon headlamp tax.
it's very common to hide things you want / need with shit you don't and make you pay for both.
BACK IN THE DAY WE COULD
You went to the dealer, talked to some bitch ass dude who was all like "pls buy the car I'm being nice to you", you asked for the options, you ticked what you wanted, and you got it.
It's still possible afaik but you need to talk to the proper manager and not just the sales person, make the order, put some down payment, and then wait like months.
At the same time, waiting and going through options list will leave you basically at the same price as the upper trim model already specced.
So, unless you want to avoid a special feature you dislike in upper trim (say, touch screen shit) just buying it as is tends to be faster and cheaper.