Chaining Multiple USB Hub Controllers for Serial Interfacing
Not really an answer, I just can't comment...

Did you evaluate the option to use simple Arduino/AVR? With Atmega128 TQFP64 you should be able to get 26ish software serials, the hardware UARTs can go as fast as 2Mbps (to connect to PC host), Atmega128 has 2. 

There is also STM32F103 series ARM with 72MHz clock.

On the PC host side, if you really need 26 separate COMs, there is possibility to use virtual COM ports...some software will be needed to "aggregate" data of these 26 ports into real 1. Or edit your PC software. Creating some custom protocol (similar to Modbus) to address each serial port shouldn't be hard. Same on MCU side.