I would like to see a button in INV-B to show lot availability of an item that has lot control.

With the Drill Downs in Evo you can add it to any place there is an inventory lookup.
 
 You just need to use SU-A to create the lot grid, then SU-B to add the drill down link.
 
 

I just added it in 2 mins to the SU-B program. I can email it to you it is ISDRILLM.B or you can do it yourself... You need to be on the latest Evo release (10/31/06)  
 
  Run SU-B from the queries menu.  select the Drill button. Press the Add button to add a new drill down, as the parent select the Inventory grid, then for the child select the LOT grid, use the mtlot.code as the index.  Type in the menu text for the drop down (lot availability) then select the Source field in this case the BKIC.PROD.CODE and in the Target field select MTLOT.CODE.  Save and you are ready to go. You now have a new option in the drill down (green down arrow) button on all the inventory lookups for Lot Availability.