Not a good plan. The way crystal malts are made, they're completely non-enzymatic and mostly unfermentable (unless a base adds enzymes to chop the larger carbohydrates back up).
Plus at 100% you're not gonna get much sugar extraction out of it.
Recipe for bad beer.
Now, there are uses for a 100% crystal malt wort, but it's more for analytical purposes than actual brewing.
Pretty easy to test this with 1lb C20, 1 gal water, US-05. Overnight cold steep, pitch the yeast dry.
When I did it (smaller batch size, C60L) , it fermented .
Didn't taste or bottle it.
There are some interesting variations: rather than cold steep,
steep the C20 for 30 minutes at 150* F
'mash' the C20 with enzymes for 30 minutes at 150* F