Roy,

The 9-bit control word is a bit of a pain, but although it is not
obvious, it is not too hard to use a standard 8-bit SPI-type
interface to communicate with the Si4734. The technique is used
in this software:

http://www.silabs.com/Support%20Documents/Software/AN264SW.zip

(See the CONTROL_WRITE and CONTROL_READ macros in file
F320_Si470x_Interface.h. Note, this code is for the Si470x parts,
but works the same for Si473x if you change the chip-address to
0xA0 instead of 0x60.)

The basic idea is to do a 2-byte read or write by packaging the
25-bit command+response or command+data in four 8-bit transactions.
(While manually banging the SENB signal before and after the
32-bits.) The first 25 bits are as documented for the part. The
last 7 bits just hang on as partial extra response or data. When
SENB is driven high, the last partial transaction is aborted and
the control interface state-machine is reset for a new transaction.

Perhaps you can simplify your interface with this technique.

Cheers,
Scott