Sharing SDRAM on Stratix II DSP Development kit

I would like to implement a design that shares the external SDRAM that
is installed on the Stratix II DSP Development Kit board between the
Nios II controller and custom circuitry that will occupy a portion of
the remaining Stratix II LE's (ALM's).

Half of the SDRAM would be dedicated for collecting interleaved 8-bit
data samples between the two on-board A/D's.  Four of the 8-bit data
samples would be accumulated from each A/D and stored, in an
interleaved fashion, into a custom-designed FIFO forming a 64-bit word.
 These 64-bit words would then be written to the SDRAM by either the
Nios II processor or a DMA engine @ 12.5 MHz.  Once the SDRAM has been
filled, the Nios II processor could be notifed (via a register?), where
the data samples stored on the SDRAM can be copied and placed into a
file in the on-board Compact Flash.

The other half of the SDRAM would be used as the program/data memory
for the Nios II processor.

Can the SDRAM be shared by using the Avalon switching fabric to allow
the functionality described? Can SOPC Builder be used to ensure that
only half of the available on-board SDRAM is used for the Nios II
leaving the rest available for storing of the A/D samples?

Any feeback would be would be appreciated.