I have hit a brick wall when trying to create a Cross-Site collection lookup
column.

When you create a custom site column to extend the OOTB Lookup column, you
are restricted to selecting lists in any site under a site collection.

To get over this constraint, I created a custom column of type
SPFieldMultiColumn, and now my column is able to retrieve values from any
list in any site across site collections.

But the problem is, I am not able to replicate the lookup functionality, i.e,

1. The user creates a new item in the destination list in Site A selects a
value in the column (ID:5 and Value:Test) and saves the item.
2. Now I go to the source list in Site B and change the item (ID: 5 and
Value:Test) to (ID:5 and Value:Testing).
3. When I have go back to Site A and view allitems.aspx of the destination
list, i see that the value is still "Test" and not the updated "Testing"

But if you set up a lookup column it is automatically updated whenever the
source value changes...

Anyone of you know how this is done...., any suggestions/ideas is greatly
appreciated...