IO540
You are absolutely right about the KLN94B and the KLB90B. In fact both devices resolve altitude in 100 foot increments from an external source and it is this which is displayed by default. This explains the need to enter or confirm the QNH on startup.
However the business of which altitude is displayed is not key to question of BRNAV certification.
BRNAV is effectively a European manifestation of RMP-5. Mostly this deals with required track accuracy, but where this accuracy is met by GPS receiver, it requires a minimum of either:
4 Satellites to be received, or
3 satellites plus an external altitude source.
The reason for this is nothing to do with giving you an accurate altitude readout. Rather, with only 3 satellites, your altitude provides the only acceptable means of unambiguously determining your position. Cast your mind back to your GPS notes and the intersecting spheres of position that are involved in position determination.
BRNAV since it permits flight by reference to GPS is therefore concerned with belt-and-braces nav capability, and the installation standards were set accordingly. Therefore, your GPS requires an external altitude source to be RMP-5/BRNAV compliant. This has nothing to do with the displayed altitude which is something of a red herring.
The BRNAV standards document is to be found on the Eurocontrol site, although it takes a little hunting down. Each member country published their own documents specifying required avionics fits.
2D