You should be able to pass symbols into constructor arguments of slim tables.

|import|
|fitnesse.slim.test|

|script|test slim|
|$X=|echo int|99|

|script|test slim|$X|
|check|return constructor arg|99|

|script|
|start|test slim|$X|
|check|return constructor arg|99|

|test slim|$X|
|return constructor arg?|
|99|