Hi there.
Lap belts for harnessed seats are easy and fine. However, there are a couple of risks. First, you have no top tethers, and you cannot get top tethers. Ideally, you'd rear face until booster maturity. What seat do you have, and how tall and heavy is your child?
Second, in a year or two when your child is booster ready, this car can't really be used in any optimal fashion. As you've mentioned, there are no shoulder belts in the back. You cannot use a booster seat without a shoulder belt. When the time comes, you may end up prematurely having to move your child to the front seat.
Is this your daily driver, or a fun car?
Wendy