I found this when I searched for the answer to the OP. Here it is and hope it helps:
''Dear Bhagavathas,
1) I have few questions regarding sandhyavandanam.
How to perform sandhyavandanam during inconvenient times like long distance
travelling, bed-ridden illness and hospitalization
Reply:
Performance of Sandhyavandanam can never be skipped. In the circumstances
stated by you, there are ways of observing this Nityakarma.
i). During long distance travel, You may do jala bhAga also with tender
coconut water (if available and no other water is available) and if you are
unable get any water at all, you may do the jala bhAga by "mAnasikam". You
may recite the mantras in your mind itself like in a whisper to be heard by
you only. And, when you reach a place where you can get water, you may do
the regular Jala and Japa bhAgams with a Prayschitta sankalpam " PoorvEdhyu:
ananushTitham PrAthah / mAdhyAnika /sAyam sandhyAm (as the case may be)
upAsishyE".
ii). During prolonged illness etc. You can ask someone else to do it for
you. Similarly, you may help someone bedridden by doing it on his behalf.''
Source: Re: Sandhyavandanam