It doesn' t *look* like you can...
 
 McFortiGate (24) # sh
 config firewall policy
     edit 24
         set srcintf " web-proxy" 
         set dstintf " wan1" 
         set srcaddr " all" 
         set dstaddr " all" 
         set action accept
         set schedule " always" 
         set service " proxy_all" 
     next
 end
 
 McFortiGate (24) # set ?
 *srcintf                      Source interface name.
 *dstintf                      Destination interface name.
 srcaddr                      Source address name.
 dstaddr                      Destination address name.
 rtp-nat                      Enable/disable use of this policy for RTP NAT.
 action                       Policy action.
 status                       Enable/disable policy status.
 identity-based               Enable/disable identity-based policy.
 schedule                     Schedule name.
 service                      Service name.
 utm-status                   Enable AV/web/ips protection profile.
 logtraffic                   Enable/disable policy log traffic.
 logtraffic-start             Enable/disable policy log traffic start.
 webcache                     Enable/disable web cache.
 web-auth-cookie              Enable/disable Web authentication cookie.
 comments                     comments
 block-notification           Enable/disable block notification.
 webproxy-forward-server      Web proxy forward server.
 tags                         Applied object tags.
 replacemsg-override-group    Specify authentication replacement message override                                                                                                                                                              group.
 srcaddr-negate               Enable/disable negated source address match.
 dstaddr-negate               Enable/disable negated destination address match.
 service-negate               Enable/disable negated service match.
 timeout-send-rst             Enable/disable sending of RST packet upon TCP session expiration.
 
 McFortiGate (24) # end
 
 McFortiGate #
 
 
 
 You could set an inbandwidth or outbandwidth limit on the outgoing interface itself, or else route the proxy traffic through to a second VDOM and enable a shaper on the policy governing traffic through THAT VDOM, but AFAIK, that' s about it.
					
				
			
			
				
	Regards,
 Chris McMullan
Fortinet Ottawa