It is not going to work with LEDs that are designed to work at more than a few mA, which your 12V strips probably are.

The 48V from a mixer is only designed to supply very small amounts of current, just enough to bias a condenser mic and power a single FET or valve preamp. The main 48V from the central PSU is distributed to a number of mic channels (anything up to 60 or so) and there are usually distribution resistors, a few kilohms or so, to ensure that a faulty cable or mic won't take out the whole system.

I know that some manufacturers put lights in their mics, but they must be making sure that the current they are drawing is no more than 1 or 2mA. That 30 or 40mA is for the whole desk, if you pull that on a single channel you will likely not get anything like 48V. Possibly the mic may not function correctly. Even if it works on your Mackie, you will have issues plugging into other desks.

EDIT: by request. It's hard to find decent schemas of mic channels (where the 48V is used) but [here is one][1] - the 48V switch is SW1, and the 48V bus is the signal coming in from the lower left. The two distribution resistors are 6k8 in this case. So you cannot get more than 7mA. Values from about 2k to 10k are usual.

(FWIW I worked for Soundcraft back in the day, and also serviced and restored others by DDA, Neve and so on, and they all work pretty much this way. The reason is simply simplicity and also keeping the size and heat design of the central PSU sane. Because these supplies have to be ultra-quiet, they are normally linear regulators, so keeping the supply current on the low side makes sense.)



  [1]: https://i249.photobucket.com/albums/gg209/templemark/soundcraft%20500/originalcircuit2.jpg