You can just change brightness of SC2 logo in the Battle.Net UI, bring modified texture file back to MPQ-archive and it will redownload again. There are no more way to add custom backgrounds for Battle.Net, no more Stronger Team Color mod, no more BroodWar sounds, etc etc.
I will be glad, if it's wrong and there will be a way to add new backgrounds and new resources into game without modifying original resources or any other way to customize some ingame stuff.
But with patch 1.5.0 after game launcher it will lock modified file (renamed or not, no matter) and start redownload it.
^ same happens with any modified file. Also method with creating new enGB.sc2assets or base.sc2assets, created in "LibertyMulti.sc2mod" folder, no longer works.
As you can see, if file was broken or modified (for example you're deleted one DDS-texture file), instead of redownloading 2 gb sized MPQ-file, it skips entire file (that is unmodified) and starts redownload only modified part of MPQ file (at screenshot it starts redownload from 272 mb to 295 mb, and not all 295 mb from zero)
You can test it now, launch SC2 Arcade Beta 1.5.0, open any MPQ file, go to Texture folder (inside MPQ file) and just change small DDS-texture (change brightness or saturation for example, modify it in any way) and bring it back to MPQ-file, where you get it from. After launcher it will redownload file and lock modified one. Same system was implemented in Diablo 3 game client, where you can't change ingame language via renaming files (like koKR to enGB), because with that it will redownload files too. Glad they added Global Play function, that will be in HotS too.
But there are a good side of that game client revamp. PROBABLY less cheaters, maphackers, drophackers and other heretics. I don't know how their cheats works, but probably (I'm just guessing) some of them used same ways to alter game resources, like Stronger Team Color mod/BW sounds/Bnet backgrounds/etc. So after patch 1.5.0 come out, probably many cheats and hacks will be broken too.
Again, here are just my assumptions based on small tests in Arcade-Beta 1.5.0. I will be VERY glad, if I'm wrong and it will be still possible to do some ingame customization.