Well, base your virus partially on DD-WRT or any other open source router OS and you're in the game. Consumer routers usually have a button "upload new firmware" somewhere in the administrative web UI.
Ok, so you have a lot of stuff based on similar SoCs and common platforms like MIPS (although ARM is in use it tends to be more low power on consumer routers, broadcom's MIPS certainly is the major player). However most of the routers have different components, memory configurations and here's the bit that kills random firmware upgrades: Different Memory Technology Device (MTD) layouts. Think of this a little like partition layouts. The flash upgrade process will vary by vendor and will often flash MTDs of specific sizes. If the sizes don't match, it won't work or worse it'll brick the device.
Source: 2 years ago I spoke at Blackhat EU on breaking into MiFi hotspots[1] and I've been breaking Linux-based hardware devices since about 2011.
