Unless I am mistaken, I don’t think we have a holding register that can be changed from the home assistant UI directly like the modbus platform switch operates bidirectionally for a binary value.
Right now I use get and set automations to update a defined input_number box, but it is very messy compared to the modbus platform switch.
Yeah this would be an awesome feature, please implement.
We Currently have digital output in the form of the “binary_sensor” class, digital input in the “switches” class, analog output in the “sensors” class but completely missing an inbuilt analog input class.
I really miss Input number. It’s hard to believe that this is not possible.
vote !!! I miss also that feature! I cannot believe that only a few people wants to have this feature. 
Thanks for the support on the question, in all fairness it is probably a bit trickier than the switch only operates a bit.
A holding register can take many form (real, int, dint) so it is a fair bit more work probably.
If this feature would be implemented, it would be a big step forward to be an alternative to a basic scada system imo.