SFP+ ports on the current generation of Aruba switches (3810M, 5400R v3, and soon 2930F) support 100M optical transceivers (HP X111 / J9054C) . Key is the device needs to support an SGMII host interface, which our Aruba 100M devices all do.
802.1x support is agnostic to optical PMD type, so there no hardware limitations.