Populating unimplemented pads to double(normally) any given DRAM Card's capacity would work fine AFAIK. Apple's 4/8MB DRAM PCBs are likely not easily converted to higher capacities. CAS and RAS line availability is the limiting factor for converting low capacity cards and designing higher capacity cards.

Duo System DevNote: C H A P T E R 6 - DRAM Expansion Cards:

When designing a DRAM expansion card, you should include logic for address and
control circuitry, since all signals required are available at the expansion connector. There
are ten DRAM address bits (FRA0:9), and 32 data lines. Data bits 24 through 31 are
buffered (DDATA[31:24]). Other data (Data 23:0) is not buffered.

Click to expand...

Five Banks available on the Memory Card:

The 4MB expansion card houses its DRAMs in banks two (2) and three (3). Banks four (4)
and five (5) are empty. The eight-megabyte expansion card uses all four memory banks.
Banks zero through three (0-3) will support only two megabytes of DRAM each. Banks
four (4) and five (5) will support eight megabytes per bank, providing 16-Mbit DRAMs
are used. If banks two through five (2-5) are fully populated, it is possible to design an
expansion card that provides a total capacity of 24 MB. (See Figure 6-2.)
Using a different packaging technique, it is theoretically possible to expand memory
capacity up to 32 MB. Although the Apple DRAM expansion card is physically
constrained to four banks, appropriate column address and row address strobe signals
are provided to support a fifth bank. Using a special IC packaging technique, such as
tape automated bonding (TAB), an expansion card using the fifth bank could provide
an additional 8 MB of DRAM, for a total memory capacity of 3 2MB (28 MB on the
expansion card, and 4 MB on the main logic board). This configuration has not been
tested or approved by Apple Computer.

Click to expand...

Possible Complicating issues:

Buffering of Data bits 24-31

ROM limitation nastiness

Upside:

JAE Connector required is a thru hole component, readily available and easily harvested for reuse.

Modem/Power Card bay and PCB plane is contiguous with DRAM Card bay so a combo Memory/Power Card PCB is doable.

Additional surface area for custom PCB consisting of available Power Card surface area could be very handy. [}] ]'>

JAE Connector and Power Buttons on utterly useless 14.4 Modems are eminently available for harvest.

JAE Board interconnect needed for additional support/alignment integrity.

Modem Connector has its own power budget and connections, ADB hack optional! [] ]'>

WAG:

While Memory is ROM limited as in 1400 and the Compact Mac Series, any available Addressing CAS/RAS signals from Banks 1 and 2 a/o the Memory Controller itself on the Logic Board might be hijacked by patching to trace cut contacts on the Modem/Power Card's JAE board interconnect.

The equivalent of Compact Virtual on the 4MB limited SE could allow for additional DRAM to be used as Silicon Disk. mapped to unimplemented.

Question:

Might an Init/Driver be developed to allow the memory space unused in Banks 1 & 2 on the MoBo to be implemented in DRAM ICs in the Modem Bay cubic?