Containment paths are always a little tricky. In my (limited) experience, even if you can trace the path by AdminConfig.parents, you may not always be able to use getid.
Are you restricted to using getid? If not, here are some alternatives that will get you an ActivationSpec at the /Cell/Node/Server level:
Querying using AdminConfig.list
This approach will list the Activation Specifications at the specified scope (in our case, the server), and grab the one that has it's name attribute equal to 'myActiveSpecName'.
server = AdminConfig.getid('/Cell:mycell/Node:mynode/Server:myserver')
activationSpec = ''
for as in AdminConfig.list('J2CActivationSpec', server).splitlines():
if AdminConfig.showAttribute(as, 'name') == 'myActiveSpecName'
activationSpec = as
print 'found it :)'
Using Wildcards
This approah uses AdminConfig.list as well, but with a pattern to narrow down your list. If you know your activation spec's configuration begins with myActiveSpecName, then you can do the following:
activationSpec = AdminConfig.list('J2CActivationSpec', 'myActiveSpecName*')