The important part: FlexStack is the name of the stacking technology used by the Cisco Catalyst 2960-S series of fixed Ethernet switches. FlexStack is specific to Cisco Catalyst 2960-S switches, and only Cisco Catalyst 2960-S Series Switches use FlexStack.
Longer Answer: No, because they use different stacking protocols and different physical cabling to do so.
If I understand right, it sounds like you want to capture some traffic traversing your switch, but you want to filter that traffic before it is placed on the wire connecting to the capturing station.
I doubt you can do that with a 3560-class switch. Check this config guide. I didn't see in there any mention of VACL's within RSPAN sessions. Not that the feature couldn't possibly exist, but from what I remember, VACL's are 6500-specific and are used mainly to filter live traffic passed within any particular VLAN.
I would look at other ways of isolating the interesting traffic, unless you can RSPAN the traffic to a 6500.
Best Answer
Answer: No
Source: http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps6406/white_paper_c11-578928.html
The important part: FlexStack is the name of the stacking technology used by the Cisco Catalyst 2960-S series of fixed Ethernet switches. FlexStack is specific to Cisco Catalyst 2960-S switches, and only Cisco Catalyst 2960-S Series Switches use FlexStack.
Longer Answer: No, because they use different stacking protocols and different physical cabling to do so.