Yes, you're right. I could've looked into a bit earlier: http://linux-sunxi.org/A31#Overview (SGX544 but also featuring CerdarX as usual on Allwinner's SoCs).
But neither h.265 nor HEVC is mentioned in S500's user manual and this is all of the closed source stuff that gets installed: vd_flv1.so, vd_h263.so, vd_h264.so, vd_mjpg.so, vd_mpeg.so, vd_msm4.so, vd_rv34.so, vd_vc1.so, vd_vp6.so, vd_vp8.so and vd_xvid.so
But there's one reference inside libOMX.Action.Video.Decoder.so:
macbookpro-tk:omx tk$ strings libOMX.Action.Video.Decoder.so | grep 265
This might be a good test case regarding Actions Semi's commitment to open source. But whom to talk to?