Today diplexing will work OK even with the 5-lnb dish, but as soon as the new satellites DirecTV 10 and 11 are launched your diplexing will fail. These two satellites have transponders which generate signals in the Ka-lo band which overlaps the OTA band. So you have four options:
1) run a separate RG-6 to each location where you need OTA. Definitely the best solution if you can do it.
2) If only some of your receivers need Ka-lo, stack an old 5x8 powered multiswitch underneath the Zinwell WB68 - connect any four outputs from the WB68 to the four satellite inputs on the 5x8, connect your OTA to the fifth input of the 5x8. This will give you the four remaining outputs of the WB68 to connect to "new" receivers (currently the R20 and the HR20) and eight outputs from the 5x8 to connect to "old" receivers, including diplexed OTA.
No-one will really know if this works until the new satellites are launched. The theory is that the diplexer in the old 5x8 multiswitch will have enough isolation to strip out the Ka-lo signals. If the isolation is insufficient then inserting diplexers into the four connections from the WB68 to the old 5x8 will strip out any unwanted signals in the OTA bands so will then allow the diplexer in the old 5x8 to do its job.
3) As carl suggests, in theory by moving the b-band converters to the outputs from the multiswitch instead of putting them at the receiver will move the Ka-lo signals away from the OTA bands and allow you to diplex in OTA. Again as carl points out, until DirecTV 10 and 11 are launched no-one knows if this will work.
4) Investigate DirecTV's new single-cable solution to be released sometime next year. With this solution, all the satellite signals will be stacked at different frequencies onto a single cable. So you can use wideband splitters instead of multiswitches. There has been some suggestion that this will also be possible for OTA but I haven't got any confirmation of this. If it does handle OTA as well as satellite signals that will be great, you will only need a single cable even to multiple tuner DVRs with OTA. If the stacking system does not handle OTA, all will not be lost for those people who have two cables to their dual-tuner DVR, they can use one for the stacked satellite signal and one for OTA.