Displaying continuous data with multiple transparency values?

[gadomski]

Tl:dr; Is there a way to define a continuous colormap while still mapping multiple values to transparency?

We're using rio-cogeo (via titiler) to display COG-ified MODIS products. Some MODIS products, e.g. MOD17A3HGF, have continuous data that we'd like to display with a continuous colormap. However, these products also have multiple "fill values" that make displaying "nodata" as transparent pixels tricky. E.g. MOD17A3HGF has the following two continuous variables:

Data Sets	Meaning	Units	Date	Type	Valid Range	Scale Factor
Gpp_500m	8-day total GPP	kgC/m2	/8d	int16	0 ~ 30000	0.0001
PsnNet_500m	8-day total Net Photosynthesis	kgC/m2	/8d	int16	-30000 ~ 30000	0.0001

These int16 values also have "fill values" at the top end of their domain -- we want to display all of these values as transparent pixels:

32767 = _Fillvalue
32766 = land cover assigned as perennial salt or Water bodies
32765 = land cover assigned as barren,sparse veg (rock,tundra,desert)
32764 = land cover assigned as perennial snow,ice.
32763 = land cover assigned as "permanent" wetlands/inundated marshland
32762 = land cover assigned as urban/built-up
32761 = land cover assigned as "unclassified" or (not able to determine)

For now, we're using an intervals colormap to get close to what we want, e.g.:

[
    [
        [ 0, 3750 ], [ 247, 252, 253, 255 ]
    ],
    [
        [ 3750, 7500 ], [ 229, 245, 249, 255 ]
    ],
    [
        [ 7500, 11250 ], [ 204, 236, 230, 255 ]
    ],
    [
        [ 11250, 15000 ], [ 153, 216, 201, 255 ]
    ],
    [
        [ 15000, 18750 ], [ 102, 194, 164, 255 ]
    ],
    [
        [ 18750, 22500 ], [ 65, 174, 118, 255 ]
    ],
    [
        [ 22500, 26250 ], [ 35, 139, 69, 255 ]
    ],
    [
        [ 26250, 30000 ], [ 0, 88, 36, 255 ]
    ],
    [
        [ 30000, 32766 ], [ 0, 0, 0, 0 ]
    ]
]

Is there a way to define a continuous colormap while still mapping multiple values to transparency? If not, is this something that could be implemented? I'm happy to dig in an give it a shot if its something that's possible.