Hi Jesse,
If I am in your position, what i will do is something like this:
First, if they are directly-connected, then
ASA5505[L3-interface]-------------------[L3-interface]Cisco881
or,
If they are connected via a L2 switch,
ASA5505[L3-interface]----------[L2}Switch[L2]---------[L3-interface]Cisco881
Configure one interface of the ASA and the interface of the 881 router with /30 point-to-point ip address.
Then static routes can be configured on both sides that is redistributed to routing protocols can be configured.
Example:
ASA5505:
configure terminal
interface Gx/y
description [To Cisco 881]
nameif
security-level
ip address
route ()
...
router ospf x
redistribute static subnets
or,
router eigrp x
redistribute static metric 1 1 1 1 1
CCIE (R&S) #27666
CCSI
HP MASE