You can use Client Side code in Sage CRM workflow.  The image below shows an Opportunity that is being progressed through a workflow in Sage CRM 7.3 sp3.  In this example opportunity workflow the maximum certainty allowed for any stage has been controlled. e.g.

Lead - 5
Qualified - 10
Quote Submitted - 15
Negotiating - 20

etc.

The next image shows how in a workflow rule a individual rule action (in this case a 'Set Column Value' action) can have a script.  

In this example the certainty field has a rule defined that looks like this:

[code language="javascript"]
if (this.value >10)
{
this.value = 10;
crm.infoMessage('Maximum value of certainty at this stage is 10');
}
[/code] 

If you wish to get other code into the workflow rule screen for clientside work you can use the workflow action 'Display Message on Screen' and use a translation to call a script:

[code language="javascript"]
<SCRIPT>
//window.alert('hello world');
crm.infoMessage('hello world');
</SCRIPT>
[/code]