For other people who may be reading and wondering the same question:
Not just Hanoverian, but also Danish and Holstein name their foals after the first letter of their sire.
The Swedish have no naming preference whatsoever. Some breeders follow the sire, some come up with name starting with whatever letter they like.
Oldenburg “prefers”, but does not require, the foals to be named after the first letter of their dam’s name.
KWPN does a letter by year. For example, 2013 foals must start with the letter “I”. Next year, it will be “J”. Some breeders in KWPN will name their fillies in a rhyme to their dam’s name, in order to honor the dam. For example, my mare, Nidoeschka, was born in an “N” year, but rhymed after her mamma who was Idoeschka.
Just this year, I decided to try rhyming to honor my Hanoverian dam and so even though I registered her filly with an R name from her sire, I rhymed her name to honor her mother. This might become much harder after this mare has had a few more foals!