I've not tried this with PG Lightning Admin, but if you post a question on the AM Software forum, I suspect Tony Caduto will respond - he has been very willing to communicate with me.
A "work around" is possible with PG Admin III. First, create a sequence (simply right-click sequences, select New Sequence, and fill in the pop-up box, making sure to set the Next Val to the next available value for your table). Next, make sure your table variable is set to Not Null and Unique (i.e., a primary key). Then set the default value for this variable to nextval('public.your_seq'::text). It is my understanding from the PG Admin III documentation that this recreates the internal workings of a sequence. Works fine in my app.