Adding free form Line items when forcasting in a module

Hello - I have a client that is requesting that users (non-administrators) be able to add detailed line items, on the fly, when completing their forecast.  They will also need Time as a dimension to enter forecasted amounts for these detailed line items, and the detailed line items need to sum to the total of the account.  Finally, there will need to be formulas on some of the line items. I have tried using a numbered list with a display name property along with a module using that list to allow them to add lines with proper names, however I need to build formulas for some of the list/line items, which I do not see a way to do with a list as a dimension.   I tried to create a line item subset from that “detailed line-item naming” module, however it will not let me create a Li subset using the Display Name property (it can only use the # format for the added members, which does not help me).  It seems like adding free-form line items is a fairly common request for forcasting organizations, and I am wondering if anyone has seen this before that could help me figure out a workable solution.   Thanks.