hi gopal;
yes it is possible to build that.
initially one does have doubts about it.the main reason for doubt is we read literature that is mostly concerned with RTOS for processors.not many take into account problems typical to 8051 architecture.most RTOS are written for CISC processors which have small register set.also in microprocessors one can have independent stacks for all tasks.not in 8051.
thus if u wish to write one:
1.decide a fixed reigster set(a,b,psw,rb0 etc.)
2.decide a stack in iram space in iram.
3.on task switch swap it in and out of ram along with register set.
4.keep scheduler stack and task stack seperate.
enjoy
pranav