I don't think it's possible because on Heroku's own support page they recommend using external monitoring applications. They have likely limited it to 100 lines to prevent people from using it as their only debugging tool.
http://docs.heroku.com/logs-exceptions
Accessing your logs is intended
primarily for use in debugging a newly
uploaded application that is producing
unexpected results, such as an
exception or a 500 / FAILSAFE error.
The logs displayed will be a jumble of
results from the different servers
your app happens to be running on, and
will show only the most recent 100
lines.
If you have a site in production use
by real users, use an external
exception logging service. We can
recommend Hoptoad and Get Exceptional.
Both of these services are very simple
to set up on Heroku, and provide an
easy and structured way to look at all
your production exceptions.
You should not attempt to use the
output of the heroku logs command for
metrics or usage analysis. For that,
we recommend Google Analytics.
Alternative exception monitoring applications:
- Exceptional (http://www.getexceptional.com/)
- Hoptoadapp (http://www.hoptoadapp.com/)
There are also a few projects on Github that allow you to build an exception logging system into your application:
- Exception Notifier (http://github.com/rails/exception_notification)
- Exception Logger (http://github.com/defunkt/exception_logger)
- Tartus (http://github.com/dinsley/tartarus)
Here are a couple of Railscasts tutorials by Ryan Bates on the topic:
- Exception Notifications (http://railscasts.com/episodes/104-exception-notifications)
- Handling Exceptions (http://railscasts.com/episodes/53-handling-exceptions)
I have just checked and the first suggestion, Exceptional, is actually available as an official Heroku add-on.
as is Hoptoadapp.