Not having Bento, I don't know what a "bento-like related data" look up might look like, but you have several options you can experiment with.
You can set up a conditional valuel list where selecting a category in field 1, limits the IDs in field 2 to only those components that are members of that category.
There are also ways to use scripted techniques that narrow the list of potential matches down keystroke by keystroke.
Here's a FileMaker 11 demo file: http://www.4shared.com/file/plr_jbkk/EnhancedValueSelection.html
You should be able to convert it to 12 just by opening it from 12's file menu and it should behave the same under Filemaker 12 as it does under 11.
I apologize in advance for the fact that 4Shared no longer allows you to download without signing up for their free filesharing service. I am in the process of selecting and implementing a replacement for this file sharing service due to this recent policy change on their part.
Phil, Thank you for your response. I will certainly take a look at the script that matches key strokes.
Just to clarify my comment about "Bento-like related data" layout.
Imagine that when you want to create more than one join-table records, you click a button that brings up a new layout. The new layout presents a scrollable list (value list) of all records from the related table (right most table). Each record on the new layout could have a checkbox that will allow the user to scroll the list and select multiple records (lines). The sellections will then be linked to the left table in the Many-to-many relationship via record insertion in the join table when the auxiliary layout is dismissed.
I want to avoid having the user click on the value list fieldd of a portal, scroll the long value list to find a related record, select it, and then repeat the process for the next Insertion in the join-table portal.