You can also do @formula(y - 100 ~ 0 + X) which will do the subtraction “lazily” (e.g., not create a new column in the table but subtract 100 from every element of y when generating the response vector).
Although to be fair, I’m not 100% sure this is a better solution than doing it in the table, because with this method you’re still going to have to remember to add 100 to predicted/fitted values to get them back on the original scale. At least if you’re predicted value is called y_min_100 you have a reminder to do that 
(I don’t think predict will name the column based on the coefname but I could be mistaken).