At present, at least the GL-AR300M and GL-AR750S are partitioned for 2 MB kernels. This will soon not be sufficient as ath79-nand kernels are already at close to 2,000,000 bytes.
There has been a request from the committer that the kernel size be increased for ath79-nand to 4 MB prior to acceptance of the ath79-nand PR into upstream OpenWrt.
The GL-AR750S U-Boot source is not up to date (it does not have any commits that resolve the ARP issue of late last year), which leads me question the GL-AR300M source as well. As a result, I cannot accurately determine if the supplied U-Boot can load and boot a 4 MB kernel.
Would you be able to make a definitive statement on if the U-Boot for the GL-AR750S and that for the GL-AR300M can reliably load and boot a 4 MB kernel?
Also, is there a technical reason why the GL-AR750S U-Boot was not configured to be able to boot a NAND-resident kernel?