Hello mdfnyeye,
you can achieve this using PBR = policy-based routing
in this way you can check the source addresses of packets as thy enter the internal lan interface:
if they match a route-map they are diverted to the leased line otherwise are routed to the DSL link.
you need :
a preferred default route to the DSL line
ip route 0.0.0.0 0.0.0.0 dialer1 10
a less preferred static route to leased line
ip route 0.0.0.0 0.0.0.0 eth0 200
access-list 101 permit ip host 10.10.10.10 any
route-map pbr permit 10
match ip address 101
set interface eth0
let's suppose eth1 is your internal lan interface receiving client traffic
int eth1
ip policy route-map pbr
Hope to help
Giuseppe
