Facu,
So, my question is: Can I use where: for this funcionality or I need use select:? If you think that where: is appropiate, Do I need create a new king of index to reach that?
I added the following method in MagmaCollectionReader which should fullfill your needs:
read: attributeSymbol beginsWith: aKey ^ self read: attributeSymbol from: aKey upTo: aKey maAlphabeticalNext
This allows you to write your original select: based call as
self customers where: [:reader | reader read: #name beginsWith: firstsLetterOfName asUppercase]
CU,
Udo