Have owned and patched a DM500HD for a year... Box runs well on OpenPli 4.0 (as one would expect). Problem is it's very tiny memory, You can install OpenPli 4.0 without any problems, but you will have a hard time installing the nice extra's like Picons, few necessary plugins, channel list etc. For this reason I have modified my DM500HD to have 2 USB ports. One internally and one externally. The internal USB port had a small USB stick attached to it. Used the FlashExpander tool to increase the 'usr/var' memory leaving ample room to install my wanted stuff...
Please be aware that the 'old' V1 of the DM500HD is no longer produced (this one is capable of running OpenPli 4.0), the newer V2 has a larger memory and is *not* supported by OpenPli however... If you don't know which one you have:
DM500HD V1 (production date up to 17-7-2013) has bootloader #84
DM500HD V2 (production date > 2-8-2013) has bootloader #88
Met één been in het graf..... Helaas