While potentially difficult, it's probably possible to do a smooth selection over multiple pages by:

• Recording start selection position
• Scroll the terminal up when dragging from terminal and cursor is close to/beyond top
• Scroll the terminal down when dragging from terminal and cursor is close to/beyond bottom
• Whenever the terminal is scrolled, reset the selection position
• When copy is triggered, intercept and select the text between the start and end (probably won't work in older browsers)