If you want to get this working quickly just to experiment, here are a couple of ideas:
First, you can mislead your Pixelblaze a little…
It has no way of knowing that whatever you send out as “RGB” data actually matches what the LEDs are looking for. If you tell the Pixelblaze to send 3 bytes of RGB data to your RGBW strip, it will do so. But the strip will still be looking for 4 bytes per pixel, which means that the first byte of the next pixel from the Pixelblaze will be used as the 4th byte of the previous pixel, and on down the line.
So if you set your Pixelblaze to an appropriate RGB mode and set its pixel count (actual pixel count) + (actual pixel count / 3) to account for the extra W bytes, you can use the function in the forum post below which, given a pixel index, figures out where the r,g,b and w bytes will fall and lets you set them all independently.
Calculate your white value and call your version of rgbw2rgb() instead of regular rgb.
(The function was made to allow independent control of “W” on RGBW strips, but you should be able to get it to work for your strips just by rearranging the order of the input parameters.)
And if you’ve got a Pixelblaze output expander board handy, it does have completely open source firmware, so you can reflash it to put the W byte wherever you need it. I did something related a little while ago. Here’s the forum link: