not sure exactly what you are trying to do.
the char matrix is spitting out values between 0 and 255. if you want to convert that to float, between 0. and 1., simply convert the type of the matrix to float32:
Writing an in-depth jit.expr/exprfill tutorial has been on my "Todo Someday" list for some time now. Some of the later Jitter Recipes in Book 3 make use of it to draw geometry.
For your particular problem, exprfill can't understand the "in" because, well the exprfill is a one time action on a jit.matrix. If you need a dynamic expression with inputs, you are better off using jit.expr.
Also, if you want to fill different planes with different expressions, you need to use separate exprfill messages with a plane argument before the expression, i.e. (exprfill 0 snorm,exprfill 1 snorm).